Bug: undefined exported symbol when symbols file contain unused symbols

Version: 3.1.52 onwards

Steps to reproduce:

```
//hello.c

#include

void hello_world() {
printf("Hello, World!\n");
}

```

Where the symfile is:
```
_hello_world
_unused_symbol
```

Now in terminal, run:
```
$ emcc -fPIC -c src/hello.c -fPIC -DPIC -o src/hello.o
$ emcc -sSIDE_MODULE=2 -sEXPORTED_FUNCTIONS=@symfile -o hello.so src/hello.o
```

This should give the error:
emcc: error: undefined exported symbol: "_unused_symbol" [-Wundefined] [-Werror]
Expected:
hello.so should be compiled without any errors.

More information:

This bug is introduced in 3.1.52. With the commit 7e808f1

GCC does not generate error in such situation.
To demonstrate, if the symfile file were:

```
hello_world
unused_symbol
```

Running this in terminal would generate hello.so **without any issues**:

```
gcc -fPIC -c src/hello.c -fPIC -DPIC -o src/hello.o
gcc -shared -Wl,--retain-symbols-file=symfile -o hello.so src/hello.o
```

Is this error: "undefined exported symbol" introduced from 7e808f1
expected behavior or is it a bug? Many projects use only one symbol file
in whole project, and gcc seems to allow it.