Provide a try_new API for unsupported platforms

[euclio]

Right now, if running this code on an unsupported platform it just panics. It would be nice if there was an API to check if locks were supported and handle it gracefully.

[yoshuawuyts]

Hi, thanks for opening this! There is no precedent in the stdlib for the behavior you're describing, so I'm reluctant to go that route. In the stdlib, if you call a function which is not supported on a platform (usually when targeting tier 2 platforms or lower), that will also result in a runtime error.

I'd be open to use more restrictive cfg flags so it becomes a compile error instead. But I don't think we should expose a new API for this purpose.