x86: Use little endian packet accessors.

x86: Use little endian packet accessors.

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

Change-Id: I09aa7f1e525ad1346e932ce4a772b64bf59dc350
Reviewed-on: https://gem5-review.googlesource.com/c/13456
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>