Different -Wpoison-system-directories behavior between macOS x86_64 and macOS arm64

I have an Intel Mac and a M1 Mac, both with the same OS version (macOS 11.3), both with the same version of `emsdk` (2.0.18). `em++` with `-Wpoison-system-directories` behaves differently between the two. With the following simple source code:

```
#include

int main(int argc, const char* argv[])
{
printf("Hello world!\n");
return 0;
}
```

And the following compiler invocation:

`/usr/local/opt/emsdk/upstream/emscripten/em++ -Wpoison-system-directories a.cpp`

I get no warnings on the Intel Mac, but I get the following warning on the M1 Mac:

```
warning: include location '/usr/local/include' is unsafe for cross-compilation [-Wpoison-system-directories]
1 warning generated.
```

Looking at the verbose (`-v`) output on the two machines, the only differences are that `-I/usr/local/include` is added on the M1 Mac, and the version of `node` differs between the two (`/usr/local/opt/emsdk/node/14.15.5_64bit/bin/node` on the M1, `/usr/local/opt/emsdk/node/12.18.1_64bit/bin/node` on the Intel).

Here is the full verbose output on the M1 Mac:

```
"/usr/local/opt/emsdk/upstream/bin/clang++" -target wasm32-unknown-emscripten -DEMSCRIPTEN -fignore-exceptions -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -D__EMSCRIPTEN_major__=2 -D__EMSCRIPTEN_minor__=0 -D__EMSCRIPTEN_tiny__=18 -D_LIBCPP_ABI_VERSION=2 -Dunix -D__unix -D__unix__ -Werror=implicit-function-declaration -Xclang -iwithsysroot/include/SDL --sysroot=/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot -Xclang -iwithsysroot/include/compat -v -Wpoison-system-directories a.cpp -c -o /var/folders/0f/lpnk71n94dg4f4yqv0zxwz7c0000gn/T/emscripten_temp_kjqhgwwc/a_0.o
clang version 13.0.0 (/opt/s/w/ir/cache/git/chromium.googlesource.com-external-github.com-llvm-llvm--project 94340dd5bb23fb7c4bc7d91d5ac0608eb25660a8)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /usr/local/opt/emsdk/upstream/bin
(in-process)
"/usr/local/opt/emsdk/upstream/bin/clang-13" -cc1 -triple wasm32-unknown-emscripten -emit-obj -mrelax-all --mrelax-relocations -disable-free -main-file-name a.cpp -mrelocation-model static -mframe-pointer=none -fno-rounding-math -mconstructor-aliases -target-cpu generic -fvisibility hidden -debugger-tuning=gdb -target-linker-version 609.8 -v -fcoverage-compilation-dir=/Users/schofield/Downloads/tmp -resource-dir /usr/local/opt/emsdk/upstream/lib/clang/13.0.0 -D EMSCRIPTEN -D __EMSCRIPTEN_major__=2 -D __EMSCRIPTEN_minor__=0 -D __EMSCRIPTEN_tiny__=18 -D _LIBCPP_ABI_VERSION=2 -D unix -D __unix -D __unix__ -isysroot /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot -I/usr/local/include -internal-isystem /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1 -internal-isystem /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1 -internal-isystem /usr/local/opt/emsdk/upstream/lib/clang/13.0.0/include -internal-isystem /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten -internal-isystem /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include -Werror=implicit-function-declaration -Wpoison-system-directories -fdeprecated-macro -fdebug-compilation-dir=/Users/schofield/Downloads/tmp -ferror-limit 19 -fgnuc-version=4.2.1 -fcxx-exceptions -fignore-exceptions -fexceptions -fcolor-diagnostics -iwithsysroot/include/SDL -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /var/folders/0f/lpnk71n94dg4f4yqv0zxwz7c0000gn/T/emscripten_temp_kjqhgwwc/a_0.o -x c++ a.cpp
clang -cc1 version 13.0.0 based upon LLVM 13.0.0git default target x86_64-apple-darwin20.4.0
warning: include location '/usr/local/include' is unsafe for cross-compilation [-Wpoison-system-directories]
ignoring nonexistent directory "/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1"
ignoring nonexistent directory "/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
/usr/local/include
/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/SDL
/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/compat
/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1
/usr/local/opt/emsdk/upstream/lib/clang/13.0.0/include
/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
1 warning generated.
"/usr/local/opt/emsdk/upstream/bin/wasm-ld" -o a.out.wasm /var/folders/0f/lpnk71n94dg4f4yqv0zxwz7c0000gn/T/emscripten_temp_kjqhgwwc/a_0.o -L/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libgl.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libal.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libhtml5.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libc.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libcompiler_rt.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libc++-noexcept.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libc++abi-noexcept.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libdlmalloc.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libc_rt_wasm.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libsockets.a -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --allow-undefined --strip-debug --export main --export emscripten_stack_get_end --export emscripten_stack_get_free --export emscripten_stack_init --export stackSave --export stackRestore --export stackAlloc --export __wasm_call_ctors --export fflush --export __errno_location --export-table --export __start_em_asm --export __stop_em_asm -z stack-size=5242880 --initial-memory=16777216 --no-entry --max-memory=16777216 --global-base=1024
"/usr/local/opt/emsdk/upstream/bin/wasm-emscripten-finalize" --minimize-wasm-changes --dyncalls-i64 a.out.wasm -o a.out.wasm --detect-features
"/usr/local/opt/emsdk/node/14.15.5_64bit/bin/node" /usr/local/opt/emsdk/upstream/emscripten/src/compiler.js /var/folders/0f/lpnk71n94dg4f4yqv0zxwz7c0000gn/T/tmp3nc0jxaa.txt
"/usr/local/opt/emsdk/upstream/bin/llvm-objcopy" a.out.wasm a.out.wasm --remove-section=.debug* --remove-section=producers
```

Here is the full verbose output on the Intel Mac:

```
"/usr/local/opt/emsdk/upstream/bin/clang++" -target wasm32-unknown-emscripten -DEMSCRIPTEN -fignore-exceptions -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -D__EMSCRIPTEN_major__=2 -D__EMSCRIPTEN_minor__=0 -D__EMSCRIPTEN_tiny__=18 -D_LIBCPP_ABI_VERSION=2 -Dunix -D__unix -D__unix__ -Werror=implicit-function-declaration -Xclang -iwithsysroot/include/SDL --sysroot=/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot -Xclang -iwithsysroot/include/compat -v -Wpoison-system-directories a.cpp -c -o /var/folders/0f/lpnk71n94dg4f4yqv0zxwz7c0000gn/T/emscripten_temp_q25h2mff/a_0.o
clang version 13.0.0 (/opt/s/w/ir/cache/git/chromium.googlesource.com-external-github.com-llvm-llvm--project 94340dd5bb23fb7c4bc7d91d5ac0608eb25660a8)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /usr/local/opt/emsdk/upstream/bin
(in-process)
"/usr/local/opt/emsdk/upstream/bin/clang-13" -cc1 -triple wasm32-unknown-emscripten -emit-obj -mrelax-all --mrelax-relocations -disable-free -main-file-name a.cpp -mrelocation-model static -mframe-pointer=none -fno-rounding-math -mconstructor-aliases -target-cpu generic -fvisibility hidden -debugger-tuning=gdb -target-linker-version 609.8 -v -fcoverage-compilation-dir=/Users/schofield/Downloads/tmp -resource-dir /usr/local/opt/emsdk/upstream/lib/clang/13.0.0 -D EMSCRIPTEN -D __EMSCRIPTEN_major__=2 -D __EMSCRIPTEN_minor__=0 -D __EMSCRIPTEN_tiny__=18 -D _LIBCPP_ABI_VERSION=2 -D unix -D __unix -D __unix__ -isysroot /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot -internal-isystem /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1 -internal-isystem /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1 -internal-isystem /usr/local/opt/emsdk/upstream/lib/clang/13.0.0/include -internal-isystem /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten -internal-isystem /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include -Werror=implicit-function-declaration -Wpoison-system-directories -fdeprecated-macro -fdebug-compilation-dir=/Users/schofield/Downloads/tmp -ferror-limit 19 -fgnuc-version=4.2.1 -fcxx-exceptions -fignore-exceptions -fexceptions -fcolor-diagnostics -iwithsysroot/include/SDL -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /var/folders/0f/lpnk71n94dg4f4yqv0zxwz7c0000gn/T/emscripten_temp_q25h2mff/a_0.o -x c++ a.cpp
clang -cc1 version 13.0.0 based upon LLVM 13.0.0git default target x86_64-apple-darwin20.4.0
ignoring nonexistent directory "/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1"
ignoring nonexistent directory "/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/SDL
/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/compat
/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1
/usr/local/opt/emsdk/upstream/lib/clang/13.0.0/include
/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
"/usr/local/opt/emsdk/upstream/bin/wasm-ld" -o a.out.wasm /var/folders/0f/lpnk71n94dg4f4yqv0zxwz7c0000gn/T/emscripten_temp_q25h2mff/a_0.o -L/usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libgl.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libal.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libhtml5.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libc.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libcompiler_rt.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libc++-noexcept.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libc++abi-noexcept.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libdlmalloc.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libc_rt_wasm.a /usr/local/opt/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/libsockets.a -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --allow-undefined --strip-debug --export main --export emscripten_stack_get_end --export emscripten_stack_get_free --export emscripten_stack_init --export stackSave --export stackRestore --export stackAlloc --export __wasm_call_ctors --export fflush --export __errno_location --export-table --export __start_em_asm --export __stop_em_asm -z stack-size=5242880 --initial-memory=16777216 --no-entry --max-memory=16777216 --global-base=1024
"/usr/local/opt/emsdk/upstream/bin/wasm-emscripten-finalize" --minimize-wasm-changes --dyncalls-i64 a.out.wasm -o a.out.wasm --detect-features
"/usr/local/opt/emsdk/node/12.18.1_64bit/bin/node" /usr/local/opt/emsdk/upstream/emscripten/src/compiler.js /var/folders/0f/lpnk71n94dg4f4yqv0zxwz7c0000gn/T/tmpqw8e916y.txt
"/usr/local/opt/emsdk/upstream/bin/llvm-objcopy" a.out.wasm a.out.wasm --remove-section=.debug* --remove-section=producers
```