libpayload: Add faster, architecture specific versions of memset and memcpy.

Change-Id: I0f3a82de860fd3afa10a557b37fb90fe6b06ae90
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: http://review.coreboot.org/1726
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Tested-by: build bot (Jenkins)

payloads/libpayload/arch/i386/Makefile.inc

@@ -31,6 +31,7 @@ head.o-y += head.S
 libc-y += main.c sysinfo.c
 libc-y += timer.c coreboot.c util.S
 libc-y += exec.S virtual.c
+libc-y += string.c
 
 # Multiboot support is configurable
 libc-$(CONFIG_MULTIBOOT) += multiboot.c