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

[Alexander Richardson via cfe-commits]

arichardson wrote:

It looks like there are also shorter feature names that can be used for the `UNSUPPORTED:` cases: https://github.com/llvm/llvm-project/blob/main/libcxx/utils/libcxx/test/features/platform.py

`darwin` and `windows` work (without the `system-` prefix.)

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