Auto merge of #9322 - jameshilliard:split-host, r=joshtriplett

Configure hosts separately from targets when --target is specified.

This prevents target configs from accidentally being picked up when cross compiling from hosts that have the same architecture as their targets.

closes #3349

diff --cc src/cargo/core/features.rs
Simple merge

diff --cc src/cargo/util/config/target.rs
Simple merge

diff --cc src/doc/src/reference/unstable.md
Simple merge

diff --cc tests/testsuite/build_script.rs
Simple merge