Auto merge of #7421 - ehuss:build-std-sysroot, r=alexcrichton

Auto merge of #7421 - ehuss:build-std-sysroot, r=alexcrichton

Change build-std to use --sysroot

This transitions build-std to use `--sysroot` instead of `--extern`. This is necessary because existing crates have a certain expectation of how standard library crates are exposed. It was intended that explicit dependencies in `Cargo.toml` would solve this problem, but I didn't really consider this would be a backwards-incompatible change, so using `--sysroot` is probably the best way to go, even though it's not ideal.

Closes rust-lang/wg-cargo-std-aware#31
Closes rust-lang/wg-cargo-std-aware#40