util: Undef M5OP at the end of assembly files that use M5OP_FOREACH.

util: Undef M5OP at the end of assembly files that use M5OP_FOREACH.

This may not be necessary since M5OP is defined/used at the end of the
function, but it's best to clean up after ourselves.

Change-Id: I524d92cb8dc44c6004dfa8109f3b17f56dba763e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27206
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>