arch, cpu: Add a takeOverFrom method for switching ISAs

arch, cpu: Add a takeOverFrom method for switching ISAs

This will be used by architectures to handle the m5.switchCpus at the
ISA level since some ISA specific fields might need to be aware of the
TC change.

Change-Id: If8d50c5c80bc3458d5f1d14cf93ae107314c98cf
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27712
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>