[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/backend/a64/a64_function.cc

@@ -0,0 +1,48 @@
+/**
+ ******************************************************************************
+ * Xenia : Xbox 360 Emulator Research Project                                 *
+ ******************************************************************************
+ * Copyright 2026 Ben Vanik. All rights reserved.                             *
+ * Released under the BSD license - see LICENSE in the root for more details. *
+ ******************************************************************************
+ */
+
+#include "xenia/cpu/backend/a64/a64_function.h"
+
+#include "xenia/cpu/backend/a64/a64_backend.h"
+#include "xenia/cpu/processor.h"
+#include "xenia/cpu/thread_state.h"
+
+namespace xe {
+namespace cpu {
+namespace backend {
+namespace a64 {
+
+A64Function::A64Function(Module* module, uint32_t address)
+    : GuestFunction(module, address) {}
+
+A64Function::~A64Function() {
+  // machine_code_ is freed by code cache.
+}
+
+void A64Function::Setup(uint8_t* machine_code, size_t machine_code_length) {
+  machine_code_ = machine_code;
+  machine_code_length_ = machine_code_length;
+}
+
+bool A64Function::CallImpl(ThreadState* thread_state, uint32_t return_address) {
+  auto backend =
+      reinterpret_cast<A64Backend*>(thread_state->processor()->backend());
+  auto thunk = backend->host_to_guest_thunk();
+  if (!thunk || !machine_code_) {
+    return false;
+  }
+  thunk(machine_code_, thread_state->context(),
+        reinterpret_cast<void*>(uintptr_t(return_address)));
+  return true;
+}
+
+}  // namespace a64
+}  // namespace backend
+}  // namespace cpu
+}  // namespace xe