cargo update, recent rustc fixes, allow custom targets with embedded "."

[evq]

This PR is related to the changes in rust-lang/cargo#6255, originally discussed in rust-lang/rust#55041 (comment)

It allows for building using custom targets specs that have an embedded . with xargo.

I also had to make a few changes to get xargo to compile under the latest stable / nightly.

[thejpster]

If you haven't seen, @phil-opp has forked Xargo to make https://github.com/rust-osdev/cargo-xbuild. That seems to be supported currently, while Xargo is in maintenance mode.

[evq]

@thejpster there seems to be incompatibility between the way that cargo xbuild sets TARGET and how cortex-m-rt expects it to be passed when using a json file target. cargo xbuild sets it to the full file path which breaks the prefix comparisons and thus linking with the bin files. I'm not sure if this is an issue that should be resolved in cortex-m-rt or cargo-xbuild.

edit: I see you ran into similar issues re - rust-embedded/cortex-m-rt#145

[RalfJung]

We have updated xargo for recent rustc separately and also just landed a cargo update, so I will close this PR.

Feel free to re-submit the part about embedded . -- but unfortunately the entire thing is just one commit so it is not clear which changes are relevant for that.