[libclc][CMake] Use clang/llvm-ar on Windows (#186726)
When LLVM_TARGETS_TO_BUILD contains host target, runtime build sets
CMAKE_C_COMPILER to clang-cl on Windows.
Changes to fix build on Windows:
- libclc struggles to pass specific flags to clang-cl MSVC-like interface.
- compile flag handling will be consistent across all host systems.
- libclc build is cross-compilation for offloading targets.
