MINOR: debug: make sure calls to ha_crash_now() are never merged

MINOR: debug: make sure calls to ha_crash_now() are never merged

As indicated in previous commit, we don't want calls to ha_crash_now()
to be merged, since it will make gdb return a wrong line number. This
was found to happen with gcc 4.7 and 4.8 in h3.c where 26 calls end up
as only 5 to 18 "ud2" instructions depending on optimizations. By
calling DO_NOT_FOLD() just before provoking the trap, we can reliably
avoid this folding problem. Note that this does not address the case
where abort() is used instead (DEBUG_USE_ABORT).