x86, smap: Add CR4 bit for SMAP

Add X86_CR4_SMAP to <asm/processor-flags.h>.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/r/1348256595-29119-4-git-send-email-hpa@linux.intel.com