[clang][ARM] Re-word PACBTI warning.

The original warning added in D115501 when pacbti is used with an
incompatible architecture was not exactly correct because it was
not really ignored and can affect codegen.

Therefore reword to say that the pacbti option is incompatible with
the given architecture.

Reviewed By: chill

Differential Revision: https://reviews.llvm.org/D119166