Rework check for libclang_rt to see if the needed library exists.

Currently libclang_rt is not provided for cross-building and as such
is not connected to cross-tools. For building clang once in universe
it is likely that libclang_rt won't exist for the universe toolchain
but even if it did it would not support anything but the native arch.
So explicitly check for support before enabling h_raw.

MFC after: 1 week
Reviewed by: dim
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D16012