[ARM] Add enhanced counter virtualization system registers

Summary:
This patch upstreams support for the ARMv8.6A Enhanced Counter Virtualization
(ECV) extension, which adds 6 new system registers.

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

Reviewers: t.p.northover, rengolin, SjoerdMeijer, pcc, ab, chill

Reviewed By: SjoerdMeijer

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77094