m68k: use CPU_HAS_NO_BITFIELDS for signal functions

When reworking bitops.h to be clean for all processor types we introduced
a CONFIG_CPU_HAS_NO_BITFIELDS define to signal whether this processor type
supported the bit field instructions. The ARCH_SIG_BITOPS functions for
m68k use these instruction types. We should base the use of these functions
(or the generic versions) on the CONFIG_CPU_HAS_NO_BITFIELDS define.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>