Update the UI around using crates.io

Try to purge the word "registry" as much as possible in favor of just referring
to crates.io as "crates.io". This also enables the default index as being the
official rust-lang crates.io index.
Fix OUT_DIR not being passed to rustdoc
The bundled gcc has moved locations
auto merge of #857 : alexcrichton/cargo/update, r=alexcrichton

We don't want to use the bundled gcc and the bundling location has changed since when this was originally written.
auto merge of #844 : alexcrichton/cargo/less-registry, r=brson

Try to purge the word "registry" as much as possible in favor of just referring
to crates.io as "crates.io". This also enables the default index as being the
official rust-lang crates.io index.