Fixes #6874.

Use runtime.GC() as a stronger version of runtime.Gosched() which tends to bias the running goroutine in an otherwise idle system. This appears to reduce the worst case number of spins from 600 down to 30 on my 2 core system under high load.

LGTM=iant
R=golang-codereviews, lucio.dere, iant, dvyukov
CC=golang-codereviews
https://golang.org/cl/56540046