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/goalc/test_vector_float.cpp

@@ -51,7 +51,7 @@ class WithMinimalGameTests : public ::testing::Test {
   void TearDown() {}
 
   struct SharedCompiler {
-    SharedCompiler(GameVersion v) : compiler(v) {}
+    SharedCompiler(GameVersion v) : compiler(v, emitter::InstructionSet::X86) {}
     std::thread runtime_thread;
     Compiler compiler;
     GoalTest::CompilerTestRunner runner;