Auto merge of #5322 - matklad:correctly-use-unstable-options, r=alexcrichton

Auto merge of #5322 - matklad:correctly-use-unstable-options, r=alexcrichton

Properly use unstable options for out-dir

@alexcrichton how exactly are unstable CLI options supposed to be handled?

One can do `-Z unstable-options my-opt` (this is done for `registry`, and this pr uses the same approach for `out-dir`). Once can also do `-Z my-opt`. Doc comments say that `-Z my-opt=val` is also possible, but in reality it does not work.

This infra was inherited from `rustc`, which is a slightly different use-case, because it does not have subcommands. That is, if you do `-Z my-opt`, it is available for all subcommands, but does not show up in the help...

What do you think about having only `-Z unstable-options` to unlock all cli options? Or do we require a finer-graind granularity?

Somewhat related, we have a bunch of unstable options already... Do we have tracking issues for them, to know when the time comes to graduate them to stable?