Add (opt-in) vDSO support for getrandom?

[newpavlov]

This Go issue can be used as a reference: golang/go#69577

[sunfishcode]

I'm not familar with this feature yet, so I'd appreciate any guidance from anyone who is is.

At a quick glance at the glibc patch, it has special handling in pthread_create and fork. If rustix has its own implementation, it's not immediately clear how rustix should handle this, since it isn't in charge of thread creation and cleanup.

One option would be for rustix to just have a minimal wrapper around the vDSO function, rather than providing a full-service getrandom, and just require the main state management code to live elsewhere. That's relatively easy to do, and I imagine something like origin would be able to do the state management because it can assume it's in charge of all thread and process creation, however it's not clear that anything else could.