Fix CI Workflow for Building the Toolchain

[d-sonuga]

For #16.

There are three problems with the current CI workflow for building the toolchain:

• Changes made in the compiler's bootstrapping code require a git remote that points to rust-lang/rust present in CI builds.
• Only 16 commits are fetched. This is a problem because, in the compiler's bootstrap code, getting the right version of LLVM requires a retrieval of a git SHA, and getting this SHA requires a retrieval of the merge-base of the local rust repo and the upstream rust repo. The merge-base could be arbitrarily far in the git history, requiring a need to fetch the entire history.
• The default C compiler toolchain on macOS doesn't have a wasm target.

This PR resolves these problems by adding code to:

• Add a git remote that points to rust-lang/rust.
• Fetch the entire git history instead of just 16 commits.
• Install LLVM and use its clang and ar to compile on macOS instead of the default tools.