[libunwind] libunwind: Remove OS requirements from tests to make them run on more OSes (PR #167642)

[Alexander Richardson via cfe-commits]

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arichardson wrote:


This attribute is the reason the test fails on macos, since the syntax is different there. It was added in https://github.com/llvm/llvm-project/pull/85097/files. Apparently I even reviewed it but I don't recall doing so.

I think there are two options here:
1) Add an XFAIL for macos
2) Something like the following, which should work for GCC and Clang, but may not be portable to all architectures.

```suggestion
#include <stdint.h>

uintptr_t start_main;
uintptr_t end_main;

extern void foo(void);

int main(int, char **) {
    start_main = (uintptr_t)&&func_start; 
    end_main = (uintptr_t)&&func_end; 

func_start:
    foo();
    return -2;
func_end:;
}
```

https://github.com/llvm/llvm-project/pull/167642