[ARM][AArch64] Adding basic support for the v8.7-A architecture

[ARM][AArch64] Adding basic support for the v8.7-A architecture

This introduces support for the v8.7-A architecture through a new
subtarget feature called "v8.7a". It adds two new "WFET" and "WFIT"
instructions, the nXS limited-TLB-maintenance qualifier for DSB and TLBI
instructions, a new CPU id register, ID_AA64ISAR2_EL1, and the new
HCRX_EL2 system register.

Based on patches written by Simon Tatham and Victor Campos.

Reviewed By: ostannard

Differential Revision: https://reviews.llvm.org/D91772