configs: Isolate ISA related object lists

configs: Isolate ISA related object lists

Some objects are not compiled when using NULL ISA, and therefore
their object lists cannot exist.

Change-Id: I93ec576229916c892de50bb6c73cd602e18a3654
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21439
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>