sparc: Use big endian packet accessors.

sparc: Use big endian packet accessors.

We know data is big endian, so we can use those accessors
explicitly.

Change-Id: I06fe35254433b20db05f5f10d0ca29a44d47c301
Reviewed-on: https://gem5-review.googlesource.com/c/13458
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>