x86: Fix up byteorder.h for x86_64

Remove the very old x86 code and add support for 64-bit.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/include/asm/byteorder.h

@@ -8,24 +8,25 @@
 
 static __inline__ __u32 ___arch__swab32(__u32 x)
 {
-#ifdef CONFIG_X86_BSWAP
 	__asm__("bswap %0" : "=r" (x) : "0" (x));
-#else
-	__asm__("xchgb %b0,%h0\n\t"	/* swap lower bytes	*/
-		"rorl $16,%0\n\t"	/* swap words		*/
-		"xchgb %b0,%h0"		/* swap higher bytes	*/
-		:"=q" (x)
-		: "0" (x));
-#endif
+
 	return x;
 }
 
+#define _constant_swab16(x) ((__u16)(				\
+	(((__u16)(x) & (__u16)0x00ffU) << 8) |			\
+	(((__u16)(x) & (__u16)0xff00U) >> 8)))
+
 static __inline__ __u16 ___arch__swab16(__u16 x)
 {
+#if CONFIG_IS_ENABLED(X86_64)
+	return _constant_swab16(x);
+#else
 	__asm__("xchgb %b0,%h0"		/* swap bytes		*/ \
 		: "=q" (x) \
 		:  "0" (x)); \
 		return x;
+#endif
 }
 
 #define __arch__swab32(x) ___arch__swab32(x)