util: Redistribute command code in the m5 utility.

util: Redistribute command code in the m5 utility.

This division will make it easier to test both the common command code,
and the individual commands.

Change-Id: Ib7be2b93e40d07e9724443ba26784e45ad9d3b17
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27562
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>