sim: nrun: tweak init of callback endian

sim: nrun: tweak init of callback endian

Allow ports to initialize the callback endian if they want.  This will
allow delegation of the logic out of common code in the future.

Also switch from the CURRENT_TARGET_BYTE_ORDER macro to the underlying
current_target_byte_order storage since the latter has been setup by
the sim-config module based on the same macros.  This will allow the
nrun module to be moved to common building for sharing.