arch-arm: Fix Arch detection in FS if there is not bootloader

arch-arm: Fix Arch detection in FS if there is not bootloader

In case a workload is run with no bootloader we still want to be able
to provide the simulation with the correct arch version.
Without this patch every baremetal simulation will default to AArch64, which
is the default value of ArmFsWorkload.

Change-Id: I0f766167d8983cafc1fd30d054862339eb21f73f
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26606
Reviewed-by: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>