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

common/util/os.cpp

@@ -47,6 +47,7 @@ void __cpuidex(int result[4], int eax, int ecx) {
       : "0"(eax), "2"(ecx));
 }
 #else
+// TODO ARM - implement ARM64 detection, check for NEON instead of AVX
 // for now, just return 0's.
 void __cpuidex(int result[4], int eax, int ecx) {
   lg::warn("cpuid not implemented on this platform");