arm: Use a const ThreadContext * and readMiscRegNoEffect in places.

arm: Use a const ThreadContext * and readMiscRegNoEffect in places.

Unlike readMiscReg, readMiscRegNoEffect won't have any read related
side effects and so can be used on a const ThreadContext. Also, using
a const ThreadContext * in a few functions which don't actually intend
to change state makes them usable in more situations.

Change-Id: I4fe538ba1158b25f512d3cccd779e12f6c91da6c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25944
Reviewed-by: Chun-Chen TK Hsu <chunchenhsu@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>