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

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 <[email protected]>
Signed-off-by: Geert Uytterhoeven <[email protected]>
Cc: David Howells <[email protected]>
Cc: Koichi Yasutake <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>