[libc] non-local conditional branch (cbz) in aarch64/sigsetjmp.cpp

sigsetjmp.cpp in [aarch64/sigsetjmp.cpp](https://github.com/llvm/llvm-project/blob/a1e59bdc173187ec47e6ede69c99316eaee9e375/libc/src/setjmp/aarch64/sigsetjmp.cpp#L20) uses a non-local label (that of setjmp) in the cbz instruction. Compiling this on aarch64 (MacOS) leads to an error:

```
error: conditional branch requires assembler-local label. '__ZN22__llvm_libc_22_0_0_git6setjmpEP9__jmp_buf' is external.
20 | cbz w1, %c[setjmp]
| ^
:2:7: note: instantiated into assembly here
2 | cbz w1, __ZN22__llvm_libc_22_0_0_git6setjmpEP9__jmp_buf
```

Here's a minimal reproducible example:
```
void external_target(void) {}

void foo(int condition) {
asm volatile(
// ERROR: cbz requires a local label (e.g., .Lskip_target), not an
// external symbol.
"cbz w0, %c[target] \n"
"nop \n"
:
: [target] "i"(external_target)
: "w0"
);
}

int main() {
foo(0);
return 0;
}
```
Compile this with clang -target aarch64 example.c -o a

@michaelrj-google

Start with libc/src/setjmp/aarch64/sigsetjmp.cpp at the inline assembly around line 20, then reproduce the reported failure with the provided aarch64 clang command. Determine how the conditional branch can assemble on AArch64 when targeting setjmp, and verify that the file compiles successfully without the assembler error.

Written by the indexing model from the issue text.