Rustc_codegen_GCC: Progress Report #31

"This month, we continued our work towards having rustup distribution:"

"we now use the default rust mangling scheme and we now build libgccjit in the Rust CI so that we run the tests will all the latest features of rustc_codegen_gcc in the Rust CI."

"We also transferred the ownership of my GCC fork and the gccjit.rs crate to rust-lang in order to have all the dependencies of the GCC codegen owned by this organization."

Awesome progress from Antoyo and the other contributors!