arch,cpu: Add a setThreadContext method to the ISA class.

arch,cpu: Add a setThreadContext method to the ISA class.

Also remove ThreadContext pointer parameters to some of the methods in
the ISA classes.

Change-Id: I8e502b1857d299cb2e759a9734a1df4f65f31efe
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29233
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>