dev: Build the PCI device models even in NULL_ISA builds.

dev: Build the PCI device models even in NULL_ISA builds.

There are some minor ISA dependencies in the PCI device models,
specifically that they use the set<> accessors on the packet objects.
This actually compiles fine because the NULL ISA claims to be little
endian, but really these accessors should be changed to use little
endian all the time since that's what PCI is defined to use, not
the guest endianness.

The other types of accessors, specifically the ones that default to
what the guest wants, should be excluded when building NULL_ISA, and,
pending other dependencies, the NULL_ISA should no longer have an
endianness associated with it.

Change-Id: I0739122dbf67d109e7959553a1eff0239b090ca4
Reviewed-on: https://gem5-review.googlesource.com/c/13468
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>