[libunwind] [libunwind][test] set fed test to require x86 as others arch may have cross toolchain build (PR #156383)

[Nikolas Klauser via cfe-commits]

https://github.com/philnik777 approved this pull request.

LGTM assuming the CI is happy to get that green again. Though I'm not sure we want to keep the test as-is. It seems to have quite a few assumptions about the environment baked in.

https://github.com/llvm/llvm-project/pull/156383