x86, boot: Move memcmp() into string.h and string.c

Try to treat memcmp() in same way as memcpy() and memset(). Provide a
declaration in boot/string.h and by default user gets a memcmp() which
maps to builtin function.

Move optimized definition of memcmp() in boot/string.c. Now a user can
do #undef memcmp and link against string.c to use optimzied memcmp().

It also simplifies boot/compressed/string.c where we had to redefine
memcmp(). That extra definition is gone now.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Link: http://lkml.kernel.org/r/1395170800-11059-5-git-send-email-vgoyal@redhat.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/boot/compressed/string.c

@@ -1,15 +1,4 @@
 #include "misc.h"
-
-/* Avoid intereference from any defines in string_32.h */
-#undef memcmp
-int memcmp(const void *s1, const void *s2, size_t len)
-{
-	u8 diff;
-	asm("repe; cmpsb; setnz %0"
-	    : "=qm" (diff), "+D" (s1), "+S" (s2), "+c" (len));
-	return diff;
-}
-
 #include "../string.c"
 
 /* misc.h might pull in string_32.h which has a macro for memcpy. undef that */