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

[Alexander Richardson via cfe-commits]

arichardson wrote:

The problem is that we end up with a combinatoric explosion of config files for all the different ABIs that can be used.

Would you be open to having the cmake-bridge inject CMAKE_CXX_FLAGS_INIT/CMAKE_EXE_LINKER_FLAGS_INIT (which should be empty unless you use a toolchain file).

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