Let custom codegen backends opt out of LTO

[bjorn3]

For example cg_clif doesn't support LTO. Unfortunately enabling LTO makes the linker code always omit object files, so attempting to use -Clto=yes will result in a linker error. It is not practical to just not pass -Clto=yes, as many crates enable it in their Cargo.toml. I would like to be able to make cg_clif tell rustc that LTO should be disabled, even when -Clto=yes is passed. I think there should still be a warning though.

[bjorn3]

@rustbot modify labels:+A-cranelift

[michaelwoerister]

I think the entirety of LTO should be implemented in the specific backends. It makes no sense to me to have it in the backend interface.

[bjorn3]

The linker code knows that when LTO is enabled, object files should be removed from rlibs before executing the linker. It only keeps the bitcode in that case.