arch: Move many of the generic files outside an NULL guard.

arch: Move many of the generic files outside an NULL guard.

These files can be compiled successfully even if the ISA is the NULL
ISA.

Change-Id: I67133ea674f678f33b0aa1ef55af719f2869241d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34169
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>