[AArch64] Assembly Support for FEAT_GCS/FEAT_CHK

[AArch64] Assembly Support for FEAT_GCS/FEAT_CHK

This implements support for two new 2022 A-profile extensions:
- FEAT_CHK - Check Feature Status Extension
- FEAT_GCS - Guarded Control Stacks

FEAT_CHK is mandatory from armv8.0-a, but is in the hint space so
there's no clang command-line flag for it, and we only print the hint as
`chkfeat x16` at v8.9a and above, to be compatible when using a
non-integrated assembler that might not yet know about the extension.

FEAT_GCS is optional from armv9.4-a onwards. It is enabled using `+gcs`
in a clang `-march=` or `-mcpu=` option string, or using a
`.arch_extension gcs` assembly directive.

This patch includes changes by Ties Stuij, Tomas Matheson, and Keith
Walker.

Differential Revision: https://reviews.llvm.org/D145563