refactor: migrate to rules based toolchain

[thesayyn]

WIP, #444

[fmeum]

@thesayyn Sorry, I forgot about this. Is this ready for review?

[thesayyn]

@fmeum its not close to being done, but i'd really appreciate the early feedback here. I am committed to land this and some more.

[fmeum]

@thesayyn Took me a while not the general approach looks good to me.

I like to see a workaround for bazelbuild/rules_cc#277 - have you tried to upstream it?

This repo currently reuses the toolchain config from rules_cc. Is your plan to no longer do this or to also migrate the default toolchain to the new rules? Either would be fine, just curious.

[TroyKomodo]

Ran into this issue when trying to fix toolchains_llvm to respect rules_sh posfix toolchain to replace #!/bin/bash with the correct path. From my perspective merging this PR will allow me to fix nixos builds.

@thesayyn is this PR stale? Would you mind if I continue the work here?