arch-arm: Override ISA::takeOverFrom for the Arm ISA

arch-arm: Override ISA::takeOverFrom for the Arm ISA

This is fixing switcheroo tests when using a PMU/GICv3.  When you switch
cpus you usually instantiate multiple cpus at the beginning and you
switch them at runtime with the m5.switchCpus function.

Every cpu will have its own set of ThreadContexts/ISAs.
When you switch cpu/tc/isa, you need to update the tc/isa pointer
cached in the device model otherwise those will still reference
the switched out cpu.

Change-Id: I3aeee890286851189c3a8a4d378c83f32e973361
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27713
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>