[clang][ARM] Emit warnings when PACBTI-M is used with unsupported architectures

Branch protection in M-class is supported by
 - Armv8.1-M.Main
 - Armv8-M.Main
 - Armv7-M

Attempting to enable this for other architectures, either by
command-line (e.g -mbranch-protection=bti) or by target attribute
in source code (e.g.  __attribute__((target("branch-protection=..."))) )
will generate a warning.

In both cases function attributes related to branch protection will not
be emitted. Regardless of the warning, module level attributes related to
branch protection will be emitted when it is enabled via the command-line.

The following people also contributed to this patch:
- Victor Campos

Reviewed By: chill

Differential Revision: https://reviews.llvm.org/D115501