Auto merge of #46910 - alexcrichton:thinlto-default, r=michaelwoerister

rustc: Set release mode cgus to 16 by default

This commit is the next attempt to enable multiple codegen units by default in
release mode, getting some of those sweet, sweet parallelism wins by running
codegen in parallel. Performance should not be lost due to ThinLTO being on by
default as well.

Closes #45320

diff --cc src/librustc/session/mod.rs
Simple merge