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.

src/tools/tidy/src/lib.rs

@@ -57,6 +57,7 @@ fn filter_dirs(path: &Path) -> bool {
     let skip = [
         "tidy-test-file",
         "compiler/rustc_codegen_cranelift",
+        "compiler/rustc_codegen_gcc",
         "src/llvm-project",
         "library/backtrace",
         "library/stdarch",