[AArch64][ARM] add Armv8.9-a/Armv9.4-a identifier support

For both ARM and AArch64 add support for specifying -march=armv8.9a/armv9.4a to
clang. Add backend plumbing like target parser and predicate support.

For a summary of Amv8.9/Armv9.4 features, see:
https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/arm-a-profile-architecture-2022

For detailed information, consult the Arm Architecture Reference Manual for
A-profile architecture:
https://developer.arm.com/documentation/ddi0487/latest/

People who contributed to this patch:
- Keith Walker
- Ties Stuij

Reviewed By: tmatheson

Differential Revision: https://reviews.llvm.org/D138010