[libc] Avoid implicit assumptions about potentially-optimised memcpy()

Do not assume that an architecture-specific optimised memcpy() will
have the same properties as generic_memcpy() in terms of handling
overlapping regions.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/string.c

 	uint8_t *dest_bytes = ( dest + len );
 
 	if ( dest < src )
-		return memcpy ( dest, src, len );
+		return generic_memcpy ( dest, src, len );
 	while ( len-- )
 		*(--dest_bytes) = *(--src_bytes);
 	return dest;