Add spin-wait pause intrinsic for Windows x64 platform.

Change-Id: I7504370c67a3c551627c6bb7e67c65f83d88b78e

vpx_ports/x86.h

@@ -151,8 +151,8 @@ x86_readtsc(void)
     __asm__ __volatile__ ("pause \n\t")
 #else
 #if ARCH_X86_64
-/* No pause intrinsic for windows x64 */
+#define x86_pause_hint()\
-#define x86_pause_hint()
+    _mm_pause();
 #else
 #define x86_pause_hint()\
     __asm pause