Upgrade cargo-xbuild and other dependencies

Sergej Kostjucenko requested to merge aj-upgrade-deps into master

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/

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