Enable ThinLTO for rustc on `x86_64-apple-darwin` Local measurements seemed to show an improvement on a couple benchmarks, so I'd like to test real CI builds, and see if the builder doesn't timeout with the expected slight increase in build times. Let's start with x64 rustc ThinLTO, and then figure out the file structure to configure LLVM ThinLTO. Maybe we'll then try `aarch64` builds since that also looked good locally.