arch: Make endianness a property of the OS class syscalls can consume.

arch: Make endianness a property of the OS class syscalls can consume.

That way the syscall implementations won't have to find the right
endianness to use on their own, typically by referring to TheISA.

Change-Id: I186b2f419d5dbee72cc9b5abce7356f3143f0c83
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22363
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>