[Runtimes][libc] Include kernel headers when using RUNTIMES_USE_LIBC

[boomanaiden154]

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.

[michaelrj-google]

LGTM, though you should probably double check how the cmake variable definition ordering works on this. LIBC_KERNEL_HEADERS is intended to be passed by the user but it also has a default value that might not get picked up on a fresh build

[boomanaiden154]

LGTM, though you should probably double check how the cmake variable definition ordering works on this. LIBC_KERNEL_HEADERS is intended to be passed by the user but it also has a default value that might not get picked up on a fresh build

Ah, good point to check. We have to run it after HandleLibC is invoked because we need to tag this against the runtime-libc-headers target that is created there. The setup if LIBC_KERNEL_HEADERS happens pretty early in libc/CMakeLists.txt. Given runtimes/CMakeLists.txt explicitly handles ordering CMake configuration of the runtime libraries to respect layering rules, we'll have LIBC_KERNEL_HEADERS available whenever HandleLibC is invoked.