MacOS support (build and basic checks only)

[matthew-humphrey]

Build the executables and run the minimal PR checks (build and internal compiler test, build and internal sandcastle test, syntax tests).

[jptech]

I was able to get it building on my ARM Mac with a relatively small number of changes. Once the standard Linux GitHub pipeline is running smoothly, I'm hoping we can try their Mac runners. I've not used them yet, but I hope they can satisfy this use case.

[matthew-humphrey]

Nice! I have a Linux pipeline going that builds all the executables (kanagawa, sandcastle, and risc-v-elf2mem). I saw in the sample it's possible to have one build pipeline target multiple platforms. Do you want to take a stab at that? The file is in the somewhat inaccurately named .github/workflows/build-compiler.yml