null: Stop specifying an endianness in isa_traits.hh.

The NULL ISA doesn't really have an endianness. Now that the packet
accessors which consumed that endianness are gone, we can get rid of
that setting as well.

Change-Id: I8dd4c7b8236b07df4458fea377865f30141121d4
Reviewed-on: https://gem5-review.googlesource.com/c/13466
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

arch: Cleanup unused ISA traits constants

This patch prunes unused values, and also unifies how the values are
defined (not using an enum for ALPHA), aligning the use of int vs Addr
etc.

The patch also removes the duplication of PageBytes/PageShift and
VMPageSize/LogVMPageSize. For all ISAs the two pairs had identical
values and the latter has been removed.

arch: Resurrect the NOISA build target and rename it NULL

This patch makes it possible to once again build gem5 without any
ISA. The main purpose is to enable work around the interconnect and
memory system without having to build any CPU models or device models.

The regress script is updated to include the NULL ISA target. Currently
no regressions make use of it, but all the testers could (and perhaps
should) transition to it.