[x86_64] Use memory address constraint in __bswap_16s()

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

src/arch/x86_64/include/bits/byteswap.h

 
 static inline __attribute__ (( always_inline )) void
 __bswap_16s ( uint16_t *x ) {
-	__asm__ ( "rorw $8, %0" : "=g" ( *x ) : "0" ( *x ) );
+	__asm__ ( "rorw $8, %0" : "+m" ( *x ) );
 }
 
 static inline __attribute__ (( always_inline, const )) uint32_t