Rollup merge of #87260 - antoyo:libgccjit-codegen, r=Mark-Simulacrum

Rollup merge of #87260 - antoyo:libgccjit-codegen, r=Mark-Simulacrum

Libgccjit codegen

This PR introduces a subtree for a gcc-based codegen backend to the repository, per decision in https://github.com/rust-lang/compiler-team/issues/442. We do not yet expect to ship this backend on nightly or run tests in CI, but we do verify that the backend checks (i.e., `cargo check`) successfully.

Work is expected to progress primarily in https://github.com/rust-lang/rustc_codegen_gcc, with semi-regular upstreaming, like with other subtrees.