[libclc] [llvm] [libclc][CMake] Pass FORCE_POSIX_TOOLCHAIN to ExternalProject (PR #186726)

[Wenju He via cfe-commits]

wenju-he wrote:

> I figured this would be solved by the runtimes interface passing `CMAKE_AR` manually. There's the `cmake ../runtimes` case, but if the user is doing this all custom I'd expect it to be easy enough to just override.

Thanks for the suggestion, I added a new flag FORCE_POSIX_TOOLCHAIN to override is_msvc_target and bypass msvc-style tools.

https://github.com/llvm/llvm-project/pull/186726