Cygwin: _endian.h: Add AArch64 implementations for ntohl and ntohs

Add AArch64-specific implementations of __ntohl() and __ntohs() in
`winsup/cygwin/include/machine/_endian.h`.
For AArch64 targets, use the `REV` and `REV16` instructions to perform
byte swapping, with explicit zero-extension for 16-bit values to ensure
correct register semantics.

Signed-off-by: Thirumalai Nagalingam <thirumalai.nagalingam@multicorewareinc.com>

Author: thiru-mcw

winsup/cygwin/include/machine/_endian.h

@@ -26,16 +26,26 @@ _ELIDABLE_INLINE __uint16_t __ntohs(__uint16_t);
 _ELIDABLE_INLINE __uint32_t
 __ntohl(__uint32_t _x)
 {
+#if defined(__x86_64__)
 	__asm__("bswap %0" : "=r" (_x) : "0" (_x));
+#elif defined(__aarch64__)
+	__asm__("rev %w0, %w0" : "=r" (_x) : "0" (_x));
+#endif
 	return _x;
 }
 
 _ELIDABLE_INLINE __uint16_t
 __ntohs(__uint16_t _x)
 {
+#if defined(__x86_64__)
 	__asm__("xchgb %b0,%h0"		/* swap bytes		*/
 		: "=Q" (_x)
 		:  "0" (_x));
+#elif defined(__aarch64__)
+	__asm__("uxth %w0, %w0\n\t"
+		"rev16 %w0, %w0"
+		: "+r" (_x));
+#endif
 	return _x;
 }