[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.

src/xenia/emulator.cc

@@ -63,6 +63,8 @@
 
 #if XE_ARCH_AMD64
 #include "xenia/cpu/backend/x64/x64_backend.h"
+#elif XE_ARCH_ARM64
+#include "xenia/cpu/backend/a64/a64_backend.h"
 #endif  // XE_ARCH
 
 DEFINE_double(time_scalar, 1.0,
@@ -232,11 +234,17 @@ X_STATUS Emulator::Setup(
   if (cvars::cpu == "x64") {
     backend.reset(new xe::cpu::backend::x64::X64Backend());
   }
+#elif XE_ARCH_ARM64
+  if (cvars::cpu == "a64") {
+    backend.reset(new xe::cpu::backend::a64::A64Backend());
+  }
 #endif  // XE_ARCH
   if (cvars::cpu == "any") {
     if (!backend) {
 #if XE_ARCH_AMD64
       backend.reset(new xe::cpu::backend::x64::X64Backend());
+#elif XE_ARCH_ARM64
+      backend.reset(new xe::cpu::backend::a64::A64Backend());
 #endif  // XE_ARCH
     }
   }