[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

runtimes/cmake/Modules/HandleLibC.cmake

@@ -30,6 +30,7 @@ elseif (RUNTIMES_USE_LIBC STREQUAL "llvm-libc")
   check_cxx_compiler_flag(-nostdlibinc CXX_SUPPORTS_NOSTDLIBINC_FLAG)
   if(CXX_SUPPORTS_NOSTDLIBINC_FLAG)
     target_compile_options(runtimes-libc-headers INTERFACE "-nostdlibinc")
+    target_compile_options(runtimes-libc-headers INTERFACE "-idirafter${LIBC_KERNEL_HEADERS}")
   endif()
 
   add_library(runtimes-libc-static INTERFACE)