[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/CMakeLists.txt

@@ -12,7 +12,12 @@ xe_target_defaults(xenia-core)
 # All subdirectories
 add_subdirectory(base)
 add_subdirectory(cpu)
-add_subdirectory(cpu/backend/x64)
+if(XE_TARGET_X86_64)
+  add_subdirectory(cpu/backend/x64)
+endif()
+if(XE_TARGET_AARCH64)
+  add_subdirectory(cpu/backend/a64)
+endif()
 add_subdirectory(apu)
 add_subdirectory(apu/nop)
 add_subdirectory(gpu)