[libunwind] [libunwind][libc++][libc++abi] Add cross-compilation flags to tests (PR #67201)

[Alexander Richardson via cfe-commits]

arichardson wrote:

> Is there a reason why _all_ configurations shouldn't be doing this?

I think it would be good if all of them did it, the reason I didn't is that I limited it to the ones I can test locally. Seeing as there is probably CI coverage for all of the configs, should I try updating the remaining ones?

https://github.com/llvm/llvm-project/pull/67201