m68k: CROSS_COMPILE = m68k-linux-gnu-

Recent cross-compilers are called m68k-linux-gnu-gcc instead of m68k-linux-gcc

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