[ARM] Add ARMv8.6 Fine Grain Traps system registers

Summary:
This patch upstreams support for the ARMv8.6A Fine Grain Traps (FGT)
extension, which adds 5 new system registers.

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

Reviewers: t.p.northover, rengolin, SjoerdMeijer, ab, momchil.velikov

Reviewed By: SjoerdMeijer

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76991