[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.

.gitignore

@@ -92,6 +92,8 @@ node_modules/.bin/
 /scratch/
 
 /build/
+/build-arm64/
+/build-x64/
 
 # ==============================================================================
 # Local-only paths