arch: Make and use endian specific versions of the mem helpers.

arch: Make and use endian specific versions of the mem helpers.

Rather than using TheISA to pick an endian conversion function, we can
have a version defined for big and little endian and call the right one
from the ISA code.

Change-Id: I5014504968952e21abff3c5f6cbe58ca24233f33
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22373
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>