systemc: set endianess to fix build for ARM host

systemc: set endianess to fix build for ARM host

ARM architectures were not covered in the endianess #if cases, which
lead the build to fail on an arm host with message:

error The file boost/detail/endian.hpp needs to be set up for your CPU
type.

Change-Id: Id012cf37810da113174a51746e290e25138739cb
Reviewed-on: https://gem5-review.googlesource.com/c/14595
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>