runtime: avoid racing on pendingUpdates in AIX netpollBreak

runtime: avoid racing on pendingUpdates in AIX netpollBreak

Instead of calling netpollwakeup, just do the write in netpollBreak.
Use the same signaling we now use in other netpollBreak instances.

Change-Id: I53a65c22862ecc8484aee91d0e1ffb21a9e62d8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/226199
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>