sim, arch-arm: Restore capability of running without a kernel

sim, arch-arm: Restore capability of running without a kernel

The following patch:

https://gem5-review.googlesource.com/c/public/gem5/+/24283

Removed the capability of starting a gem5 simulation without
a kernel object. This patch is restoring it

Change-Id: I6d751bac386cbb250b3593bb12463140dc964ab3
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28147
Reviewed-by: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>