9ccaf080cb
This will make `-Zbuild-std` automatically build the right crates, notably not building `std` by default, which will both be useful for users and also fix the build for https://does-it-build.noratrieb.dev.
rustc_target contains some very low-level details that are
specific to different compilation targets and so forth.
For more information about how rustc works, see the rustc dev guide.