[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/cpu/testing/util.h

@@ -13,7 +13,11 @@
 #include <vector>
 
 #include "xenia/base/platform.h"
+#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
 #include "xenia/cpu/hir/hir_builder.h"
 #include "xenia/cpu/ppc/ppc_context.h"
 #include "xenia/cpu/ppc/ppc_frontend.h"
@@ -38,6 +42,8 @@ class TestFunction {
       std::unique_ptr<xe::cpu::backend::Backend> 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
       if (backend) {
         auto processor = std::make_unique<Processor>(memory.get(), nullptr);