Add hack to compile aes_ccm_tag() with -O0 for m68k for GCC8.

GCC 8 miscompiles aes_ccm_tag() for m68k with optimization level -O[12],
which results in failure in aes_ccm_selftest():

| aes_ccm_selftest: tag 0: 8 bytes @ 0x4d3e38
| 03 80 5f 08 22 6f cb fe                          | .._."o..
| aes_ccm_selftest: verify 0 failed
| ...
| WARNING: module error: built-in module aes_ccm failed its MODULE_CMD_INIT, error 5

This is observed for amiga (A1200, 68060), mac68k (Quadra 840AV, 68040),
and luna68k (nono, 68030 emulator). However, it is not for sun3 (TME, 68020
emulator) and sun2 (TME, 68010 emulator). At the moment, it is unclear
whether this is due to differences b/w 68010-20 vs 68030-60, or something
wrong with TME.

(rin)