[ARM][clang] Option b-key must not affect __ARM_FEATURE_PAC_DEFAULT

When using -mbranch-protection=pac-ret+b-key, macro __ARM_FEATURE_PAC_DEFAULT
should still have the value corresponding to a-key, because b-key is only valid
for AArch64.

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:

- Victor Campos

Reviewed By: danielkiss

Differential Revision: https://reviews.llvm.org/D115140