[Runtimes][libc] Include kernel headers when using RUNTIMES_USE_LIBC
When setting RUNTIMES_USE_LIBC=libc we pass -nostdlibinc if the compiler
supports it. This causes compilation failures when trying to build
libc++ as libc will try and reference kernel headers in its own headers
that it now cannot find. Explicitly reference the kernel headers so the
compiler can find them.
Reviewers: vonosmas, petrhosek, #reviewers-libcxx, michaelrj-google
Reviewed By: michaelrj-google
Pull Request: https://github.com/llvm/llvm-project/pull/165114