[ARM] add ARMv8.6-A Activity monitors virtualization extension

Summary:
This patch upstreams v8.6A activity monitors virtualization
assembler support, which consists of 32 new system
registers (two groups, each with 16 numbered registers).

See ARMv8.6-AMU in the Arm Architecture Reference Manual Armv8 for more
information.

Reviewers: t.p.northover, rengolin, SjoerdMeijer, ab, john.brawn, ostannard

Reviewed By: ostannard

Subscribers: LukeGeeson, dnsampaio, ostannard, kristof.beyls, hiraditya, danielkiss, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76998