Investigate using -Zbuild-std

[RalfJung]

Right now we are using the xargo approach of building a stdlib -- we create a suitable workspace, fire off a regular cargo build with some special env vars, and copy the result.

That works very well. It might make more sense to use -Zbuild-std instead, which is designed for building the stdlib. My main concern is handling the panic runtimes properly, which did cause some trouble in Miri in the past -- I am honestly not even sure if we do this correctly right now, though, so first we need some tests covering this.