Replace xbuild with cargo build-std

[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:

[athei]

Regarding the TODO: I think passing hardcoded flags to cargo as it is done in this PR is fine. I wouldn't generate/mess with a .cargo/config that a user might want to edit manually.

What is not fine is hardcoding RUSTFLAGS. As far as I see it we just set the env variable to some hardcoded value. But that should be fixed in the mentioned issue. Not here.

[ascjones]

I wouldn't generate/mess with a .cargo/config that a user might want to edit manually.

The idea would be that it would set the default hardcoded rustflags iff they are not specified by the user (like we do with the [profile.release] section. And also would ensure (since the env var overrides the value from .cargo/config) that a user supplied RUSTFLAGS is not ignored.

But that should be fixed in the mentioned issue. Not here.

Anyway I agree that this should be fixed separately with a little more thought.

[ascjones]

CI currently broken, just waiting for a new image to rebuild before merging