drop cargo-xbuild build dependency

[japaric]

cargo-xbuild requires nightly and it's only reason building the examples requires nightly.

This tool is required because, currently, there's no bare metal Cortex-A support in rustc / rustup. (See rustc --print target-list & rustup target list). If we add support for this target to rustc / rustup (reference PR) then we'll be able to build all the code in this repo using a stable toolchain.

Hopefully, we should be able get that target support in by the 1.42 beta cutoff so all code will compile on the stable 1.42 release.

[tarcieri]

That'd be great. In the meantime I think we're fine with nightly, but supporting stable would be quite nice.

[japaric]

If we add support for this target to rustc / rustup

Submitted a PR: rust-lang/rust#68253