Rollup merge of #129876 - nnethercote:more-sysroot-rustc_codegen_gcc,…

… r=Mark-Simulacrum

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.

r? ```@Mark-Simulacrum```