[ARM64] Initial commit for arm64 backend

Based entirely off existing xbyak x86 implementation and available
tests. Still needs a lot of optimization and testing on non-Windows
platforms.

So far passes all tests and boots at least some games on Windows.

third_party/ffmpeg-xenia/config.h

@@ -32,9 +32,13 @@
 #define ARCH_X86 0
 #define ARCH_X86_64 0
 
-#if defined(__aarch64__)
-  /* ARM64 (Android) */
-  #define SLIBSUF ".so"
+#if defined(__aarch64__) || defined(_M_ARM64)
+  /* ARM64 */
+  #if defined(_WIN32)
+    #define SLIBSUF ".dll"
+  #else
+    #define SLIBSUF ".so"
+  #endif
   #undef  ARCH_AARCH64
   #define ARCH_AARCH64 1
   #define HAVE_ARMV8 1
@@ -43,9 +47,15 @@
   #define HAVE_ARMV8_EXTERNAL 1
   #define HAVE_NEON_EXTERNAL 1
   #define HAVE_VFP_EXTERNAL 1
-  #define HAVE_ARMV8_INLINE 1
-  #define HAVE_NEON_INLINE 1
-  #define HAVE_VFP_INLINE 1
+  #if defined(_MSC_VER)
+    #define HAVE_ARMV8_INLINE 0
+    #define HAVE_NEON_INLINE 0
+    #define HAVE_VFP_INLINE 0
+  #else
+    #define HAVE_ARMV8_INLINE 1
+    #define HAVE_NEON_INLINE 1
+    #define HAVE_VFP_INLINE 1
+  #endif
   #define HAVE_INTRINSICS_NEON 1
   #define HAVE_AS_FUNC 0
   #define HAVE_AS_ARCH_DIRECTIVE 0