m68k: Use cc-cross-prefix

The cc-cross-prefix is new and developed on request from Geert Uytterhoeven.

With cc-cross-prefix it is now much easier to have a few default cross compile
prefixes and defaulting to none - if none of them were present.  ARCH
maintainers are expected to pick up this feature soon.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Andreas Schwab <schwab@suse.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>