Replace xbuild with cargo build-std

Sergej Kostjucenko requested to merge aj-replace-xbuild-with-build-std into master

Created by: ascjones

The build-std feature of cargo now allows to specify the panic-immediate-abort feature. See: https://github.com/rust-osdev/cargo-xbuild#alternative-the-build-std-feature-of-cargo.

So we can use that (unstable) feature instead of depending on xbuild

Produces the same size wasm output as with xbuild e.g. flipper:

image

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