Support target-specific sharing of types

[snowsignal]

Taken from our internal issue tracker.

Original issue description

As of today, writing something like the following doesn't work:

#[cfg_attr(target_vendor = "apple", typeshare)]

It would be nice to support that, or, more generally, the concept of typesharing a type for a specific target only.

[snowsignal]

I agree with the original issue but I think the solution would work better as part of the typeshare attribute itself, instead of an external attribute that wraps typeshare.