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

[Alexander Richardson via cfe-commits]

arichardson wrote:

> This would look good to me, but we seem to be failing the Windows tests!

Unfortunately I can't see what's going wrong there and I don't have an easy way of reproducing it. I'll try to add some debugging code to this PR in the coming days.

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