Fix compilation with upstream clang builtin headers.

By using -nobuiltininc and adding the clang builtin headers resource dir
to the end of the compiler header search path, we can still find headers
such as immintrin.h but find the FreeBSD version of stddef.h/stdarg.h/..
first.

This is a workaround until we are able to settle on and complete a plan
to harmonize guard macros with LLVM. We've mostly worked out this on
FreeBSD systems by removing select headers from the installed set of
devel/llvm*, but that isn't a good solution for cross build.

Submitted by: arichardson
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17002