sim: Use the system and OS to get endianness.

sim: Use the system and OS to get endianness.

This converts the syscall implementations to either use the
OS::byteOrder constant or, if that's not available, the system's
getGuestByteOrder() accessor, to determine the byte order, instead of
relying on TheISA to provide the correct accessor.

Change-Id: Idf7b02ee8d73990224ceac9a5efaec91a5ebf79f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22364
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>