LLVM Dependency Cleanup and Debian Install Script Fixes

[bushidocodes]

Resolves #11

While working on some ARM stuff, I realized that some of the dependencies for this repo got a bit messed up back in July of last year. It seems like Phani was in the process of moving all transitive dependencies over to our gwsystems org with a clean master tracking upstream and an sfbase branch containing our downstream patches. It seems like a PR (Others/llvm-rs#1) got merged, which put things out of sync, and the effort sort of petered out. As a result, one repo contained the ARM fix and another repo contained some quality of life refactors. This PR is the result of my effort to get this straightened out such that gwsystems llvm-rs and llvm-sys repos now contain all existing work and can be considered the canonical dependencies and the equivalent repos under @Others account can be potentially archived.

The main contribution of this PR is updating the Cargo.toml to use the sfbranch of our gwsystems llvm-rs repo. This contains the following changes:

• It pins the llvm-sys dependency to the sfbase branch of the gwsystems llvm-sys repo
• The ARM fixes that Phani added on Make it work on aarch64 Others/llvm-rs#2
• The various cleanup tasks in Replaced mem::uninitialized with mem::MaybeUninit Others/llvm-rs#1

If you want to confirm this, I suggest checking out the following:

• https://github.com/gwsystems/llvm-rs/tree/sfbase
• https://github.com/gwsystems/llvm-sys.rs/tree/sfbase

I also have a handful of BASH script cleanup tasks that I encountered when testing ARM stuff on a fresh Ubuntu install on a Pi 4:

• Supporting SSH and HTTPS-based clones
• Installing wget if missing
• Not prefixing with sudo if root
• Installing clang 11 in place of 9

[Others]

Nervous about regressions, but approved for now

[bushidocodes]

Going to go ahead and merge. I rebased the branch on top of the latest master and reran our tests again.