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

[Alexander Richardson via cfe-commits]

arichardson wrote:

Debugging shows that the reason this is failing on the clang-cl bots is that the CMAKE_CXX_FLAGS_INIT are using the MSVC syntax but we pass --driver-mode=g++ explicitly:

```
lit: C:\ws\src\libcxx\utils\libcxx\test\format.py:55: note: Command ["'C:/Program Files/LLVM/bin/clang-cl.exe' -xc++ nul -Werror -fsyntax-only --driver-mode=g++   /DWIN32 /D_WINDOWS /EHsc -fms-runtime-lib=dll -nostdinc++ -I C:/ws/src/build/clang-cl-dll/include/c++/v1 -I C:/ws/src/build/clang-cl-dll/include/c++/v1 -I C:/ws/src/libcxx/test/support -D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_STDIO_ISO_WIDE_SPECIFIERS -DNOMINMAX  -std=c++03"] failed with code 1: clang-cl: error: no such file or directory: '/DWIN32'
--
  | clang-cl: error: no such file or directory: '/D_WINDOWS'
  | clang-cl: error: no such file or directory: '/EHsc'

```


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