goalc: Get CodeTester tests passing on Arm64 (only targetting macOS atm) (#3290)

This PR does the following:
- Designs a mechanism by which arm64 instructions can be encoded and
emitted
- Dispatch our higher-level instruction emitting calls to either x86 or
arm64 instructions depending on what the compiler is set to (defaults to
x86)
- Bare minimum scaffolding to get the arm64 instructions successfully
executing atleast on apple silicon
- Implement enough instructions to get the codetester test suite passing
on arm

test/offline/framework/execution.cpp

@@ -104,7 +104,7 @@ OfflineTestCompileResult compile(OfflineTestDecompiler& dc,
                                  const OfflineTestWorkGroup& work_group,
                                  const OfflineTestConfig& config) {
   OfflineTestCompileResult result;
-  Compiler compiler(game_name_to_version(config.game_name));
+  Compiler compiler(game_name_to_version(config.game_name), emitter::InstructionSet::X86);
 
   compiler.run_front_end_on_file(
       {"decompiler", "config", game_name_to_all_types[config.game_name]});