[libunwind] [libcxx][libcxxabi][libunwind] Support target-specific flags for tests (PR #65517)

[Louis Dionne via cfe-commits]

ldionne wrote:

Your `.cfg.in` files can use CMake variables themselves! In fact you can even add new parameters that are only valid for that configuration of the library, for example see https://github.com/llvm/llvm-project/blob/main/libcxx/test/configs/apple-libc%2B%2B-backdeployment.cfg.in#L14-L42

Basically, everyone has their funky use case for testing the library, and I am trying to avoid adding complexity to the "main" code path every time someone has a new use case. These config files allow hiding that complexity away from the main code path.

https://github.com/llvm/llvm-project/pull/65517