runtime: converge duplicate calls to netpollBreak into one

There might be some concurrent (maybe not concurrent, just sequential but in a short time window) and duplicate calls to `netpollBreak`, trying to wake up a net-poller. If one has called `netpollBreak` and that waking event hasn't been received by epollwait/kevent/..., then the subsequent calls of `netpollBreak` ought to be ignored or in other words, these calls should be converged into one.

Benchmarks go1.13.5 darwin/amd64:

benchmark-func           time/op (old)  time/op (new)  delta
BenchmarkNetpollBreak-4  29668ns ±1%    3131ns ±2%     -89.45%

mem/B (old)  mem/B (new)  delta
154B ±13%    0B ±0%       -100%

Change-Id: I3cf757a5d6edc5a99adad7aea3baee4b7f2a8f5c
GitHub-Last-Rev: 15bcfbab8a5db51f65da01315a5880a5dbf9e028
GitHub-Pull-Request: golang/go#36294
Reviewed-on: https://go-review.googlesource.com/c/go/+/212737


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>