[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.h

@@ -0,0 +1,44 @@
+/**
+ ******************************************************************************
+ * 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. *
+ ******************************************************************************
+ */
+
+#ifndef XENIA_CPU_BACKEND_A64_A64_FUNCTION_H_
+#define XENIA_CPU_BACKEND_A64_A64_FUNCTION_H_
+
+#include "xenia/cpu/function.h"
+#include "xenia/cpu/thread_state.h"
+
+namespace xe {
+namespace cpu {
+namespace backend {
+namespace a64 {
+
+class A64Function : public GuestFunction {
+ public:
+  A64Function(Module* module, uint32_t address);
+  ~A64Function() override;
+
+  uint8_t* machine_code() const override { return machine_code_; }
+  size_t machine_code_length() const override { return machine_code_length_; }
+
+  void Setup(uint8_t* machine_code, size_t machine_code_length);
+
+ protected:
+  bool CallImpl(ThreadState* thread_state, uint32_t return_address) override;
+
+ private:
+  uint8_t* machine_code_ = nullptr;
+  size_t machine_code_length_ = 0;
+};
+
+}  // namespace a64
+}  // namespace backend
+}  // namespace cpu
+}  // namespace xe
+
+#endif  // XENIA_CPU_BACKEND_A64_A64_FUNCTION_H_