runtime/race: more precise handling of channel synchronization

It turns out there is a relatively common pattern that relies on
inverted channel semaphore:

gate := make(chan bool, N)
for ... {
        // limit concurrency
        gate <- true
        go func() {
                foo(...)
                <-gate
        }()
}
// join all goroutines
for i := 0; i < N; i++ {
        gate <- true
}

So handle synchronization on inverted semaphores with cap>1.
Fixes issue 7718.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/84880046

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