[clang] [libc] [llvm] [libc] Add support to build and run libc tests with spirv backend (PR #208129)

[Alexey Bader via cfe-commits]

bader wrote:

I'm not an expert in `libc`, so I'll let libc maintainers to review.
I see some SPIR-V specific code in CMake files. @fineg74, I suppose the compiler toolchain for SPIR-V missing some features libc build system uses. If so, could you please, open issues for known problems?

https://github.com/llvm/llvm-project/pull/208129