m68k: Use _AC() instead of #ifdef __ASSEMBLY__

m68k: Use _AC() instead of #ifdef __ASSEMBLY__ hackery when needed, remove
hackery when unused.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>