m68k: Make gcc aware that BUG() does not return

Use `__builtin_trap()' instead of `asm volatile("illegal")' in the m68k BUG()
macros (as suggested by Andrew Pinski), to kill warnings in code that assumes
BUG() does not return.

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