mem: Remove #if THE_ISA in the AbstractMemory class.

mem: Remove #if THE_ISA in the AbstractMemory class.

This used to guard the extraction of the endianness when tracing memory
accesses. Since that's now always possible even in NULL_ISA, we don't
need conditional compilation.

Change-Id: Ie5ec76f5b0f27dd4123bc0f0a4c02438bed629ad
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34499
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>