Use `git subtree` to manage LLVM libraries and musl

[sbc100]

I only recently discover this features of git but apparently it has some advantages over git submodule for our use case.

1. It allows us to pull in just a sub-directory with the upstream repo (e.g. just compiler-rt and not the whole llvm)
2. We can maintain patches in-tree.
3. We can make changes the sub-tree in the same commit out changes to emscripten itself.

• No need to for a separate repo to keep in sync
• No need to manage .gitmodules metadata.
• No need to multi-step commits. We can commit a patch to mus land and immediately use & test it in emscripten.

See WebAssembly/wasi-libc#797 for how this was done in wasi-libc.