Rustc is now optimized with BOLT on Linux (~2% cycle/walltime improvement)

I finally got around to optimize the Rust compiler on Linux with BOLT (the LLVM binary layout optimization tool) in https://github.com/rust-lang/rust/pull/116352, around a year after applying the same technique to LLVM used by rustc (https://www.reddit.com/r/rust/comments/y4w2kr/llvm\_used\_by\_rustc\_is\_now\_optimized\_with\_bolt\_on/).

It seems to improve performance across the board by cca 2%. Not as nice as the LLVM result, but not bad either.

We have also recently enabled 1 CGU compilation of rustc on Linux in https://github.com/rust-lang/rust/pull/115554, for additional \~2% wins across the board. This has reduced the binary size of the compiler by about 30 MiB, but that has since been sadly regained by the BOLT change :D