cpu: Track misc regs in vectors in the O3 CPU instruction class.

cpu: Track misc regs in vectors in the O3 CPU instruction class.

Most instructions won't actually write to misc regs, so the overhead
should be quite small, particularlly compared to the other overheads in
the O3.

Change-Id: I840d6002cc8151f91611cfcbe2bfa52acc284c0f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38388
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>