[clang][ARM] only check -mtp=cp15 for non-asm sources

This diagnostic is really to highlight lack of support for hard thread
pointers in post-RA instruction scheduling for non-armv6k+ targets;
something that isn't run for assembler sources.

Fixes: https://github.com/ClangBuiltLinux/linux/issues/1502
Link: https://lore.kernel.org/all/814585495.6773.1636629846970@jenkins.jenkins/

Reviewed By: ardb

Differential Revision: https://reviews.llvm.org/D114124