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.
For building clang once in universe I do a 'make kernel-toolchain' with a
separate objdir. This never runs 'make libraries' so lib/libclang_rt is
never compiled. Even if we connect that to cross-tools though I think
a check like done in this diff would be needed here, or globally.
Trying to resolve this failure:
```
--- h_raw.full ---
/scratch/obj/root/git/freebsd/freebsd12-amd64/tmp/usr/bin/ld: error: cannot open /scratch/obj/root/git/freebsd/freebsd12-amd64/tmp/usr/lib/clang/6.0.0/lib/freebsd/libclang_rt.ubsan_standalone-x86_64.a: No such file or directory
cc : error: linker command failed with exit code 1 (use -v to see invocation)
```