Add rustup.sh to the repo

Just to have it somewhere to point to. Updating it will not
automatically update the one on static.rust-lang.org.
Untabify rustup.sh
Long lines
auto merge of #17851 : brson/rust/rustup, r=alexcrichton

Just to have it somewhere to point to. Updating it will not
automatically update the one on static.rust-lang.org.