[build] Avoid strict-aliasing warning for gcc 4.3

Signed-off-by: Bo Yang <boyang@suse.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/include/bits/byteswap.h

 static inline __attribute__ (( always_inline )) void
 __bswap_64s ( uint64_t *x ) {
 	struct {
-		uint32_t low;
-		uint32_t high;
+		uint32_t __attribute__ (( may_alias )) low;
+		uint32_t __attribute__ (( may_alias )) high;
 	} __attribute__ (( may_alias )) *dwords = ( ( void * ) x );
 	uint32_t discard;