runtime: use fast path when channel has been closed and empty

[delichik]

update #77522

Check channel emptiness in recv regardless of blocking mode. If the channel is empty and already closed, recheck emptiness and allow recv to return immediately without entering the lock.

This improves receive performance on closed channels and also benefits cgocallback performance. The side effect is that normal blocking receives perform one additional empty() check, but this cost is negligible compared to acquiring the lock.

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