mn10300: only add -mmem-funcs to KBUILD_CFLAGS if gcc supports it

It seems the current (gcc 4.6.3) no longer provides this so make it
conditional.

As reported by Tony before, the mn10300 architecture cross-compiles with
gcc-4.6.3 if -mmem-funcs is not added to KBUILD_CFLAGS.

Reported-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Koichi Yasutake <yasutake.koichi@jp.panasonic.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>