alpha: Use little endian packet accessors.

alpha: Use little endian packet accessors.

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

Change-Id: Ieb9c1eb8a4fec31ee69cbbfd8c1afdf9f64de366
Reviewed-on: https://gem5-review.googlesource.com/c/13459
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>