Remove code for ThinLTO from cg_gcc

[bjorn3]

It was just a dummy implementation to workarround the fact that thin local lto is the default in rustc. By adding a thin_lto_supported thin local lto can be automatically disabled for cg_gcc, removing the need for this dummy implementation. This makes improvements to the LTO handling on the cg_ssa side a lot easier.

cc #rustc-codegen-gcc > thin LTO implementation
This should make the work on rust-lang/compiler-team#908 easier.

r? rust-lang/wg-gcc-backend

[rustbot]

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

rustc_macros::diagnostics was changed

cc @davidtwco, @TaKO8Ki, @JonathanBrouwer

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.