sim: Expose the system's byte order as a param

sim: Expose the system's byte order as a param

There are cases where a system's byte order isn't well-defined from an
ISA. For example, Arm implementations can be either big or little
endian, sometimes depending on a boot parameter. Decouple the CPU byte
order from the System's default byte order by exposing the System's
byte order as a parameter that defaults to big endian for SPARC and
POWER and little endian for everything else.

Change-Id: I24f87ea3a61b05042ede20dea6bb056af071d2c0
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33175
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>