arch,cpu: Change setCPU to setThreadContext in Interrupts.

arch,cpu: Change setCPU to setThreadContext in Interrupts.

The ThreadContext can be used to access the cpu if needed, and is a
more representative interface to various pieces of state than the CPU
itself. Also convert some of the methods in Interupts to use the
locally stored ThreadContext pointer instead of taking one as an
argument. This makes calling those methods simpler and less error
prone.

Change-Id: I740bd99f92e54e052a618a4ae2927ea1c4ece193
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28988
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>