Use sysroot crates maximally in `rustc_codegen_gcc`.

This shrinks `compiler/rustc_codegen_gcc/Cargo.lock` quite a bit. The
only remaining dependencies in `compiler/rustc_codegen_gcc/Cargo.lock`
are `gccjit`, `lang_tester`, and `boml`, all of which aren't used in any
other compiler crates.

The commit also reorders and adds comments to the `extern crate` items
so they match those in miri.