[ARM][clang] Define feature test macro for the PACBTI-M extension

If the extension string "+pacbti" was given in -march=... or -mcpu=... options the compiler shall define the following preprocessor macros:

    __ARM_FEATURE_PAUTH with value 1.
    __ARM_FEATURE_BTI with value 1.

This patch is part of a series that adds support for the PACBTI-M extension of
the Armv8.1-M architecture, as detailed here:

https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/armv8-1-m-pointer-authentication-and-branch-target-identification-extension

The PACBTI-M specification can be found in the Armv8-M Architecture Reference
Manual:

https://developer.arm.com/documentation/ddi0553/latest

The following people contributed to this patch:

- Momchil Velikov
- Ties Stuij

Reviewed By: miyuki

Differential Revision: https://reviews.llvm.org/D112431