arch-arm: only change the pc address when GDB registers are set

The entire pcState was being reset, which made the simulation incorrectly
switch to aarch32 from aarch64.

Change-Id: I9ba7dd0ed95bc6120d94393bba272e0cb8c081c5
Reviewed-on: https://gem5-review.googlesource.com/c/14496
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>