[AArch64] Adding the v8.7-A LD64B/ST64B Accelerator extension

This adds support for the v8.7-A LD64B/ST64B Accelerator extension
through a subtarget feature called "ls64". It adds four 64-byte
load/store instructions with an operand in the new GPR64x8 register
class, and one system register that's part of the same extension.

Based on patches written by Simon Tatham.

Reviewed By: ostannard

Differential Revision: https://reviews.llvm.org/D91775