Upgrade cargo-xbuild and other dependencies
Created by: ascjones
Fixes #70 (closed).
Upgrades to cargo-xbuild 0.6.0
which includes https://github.com/rust-osdev/cargo-xbuild/pull/87. This makes it compatible with the new rust-src
layout here https://github.com/rust-lang/rust/pull/73265.
I will merge this once there is a new nightly
> 2020-07-30 which includes rustfmt
: https://rust-lang.github.io/rustup-components-history/