Auto merge of #9118 - bjorn3:profile_codegen_backend_option, r=joshtriplett

Add a profile option to select the codegen backend

This makes it possible to compile only specific crates with a certain codegen backend.

I have tested this by compiling Bevy with cg_llvm, but one of the examples using cg_clif.

By the way I noticed that many unstable profile options are not checked when using profile overrides.