Add native memset() function on x86

Add native memset() function on x86

Change-Id: Ia118ebe0a4b59bdcefd78895141a365170f6aed2
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/737
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins)