sh: Fixup __cmpxchg() compile breakage with gcc4.

As reported by Manuel:

When I build linux with GCC-4.x and enable
CONFIG_CC_OPTIMIZE_FOR_SIZE linking fails with this error:

  LD      .tmp_vmlinux1
  kernel/built-in.o: In function '__cmpxchg_called_with_bad_pointer'
  make[1]: *** [.tmp_vmlinux1] Error 1
  make: *** [_all] Error 2

This ended up being an inlining problem, fixed by explicitly
including linux/compiler.h and grabbing the definitions from there.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>