We only need the memory barrier in poll_runtime_pollSetDeadline only
when one of the timers has fired, which is not the expected case.
Memory barrier can be somewhat expensive on some archs,
so execute it only if one of the timers has in fact fired.

name                  old time/op  new time/op  delta
TCP4OneShotTimeout-6  17.0µs ± 0%  17.1µs ± 0%  +0.35%  (p=0.032 n=5+5)
SetReadDeadline-6      232ns ± 0%   230ns ± 0%  -1.03%  (p=0.000 n=4+5)

Update #25729

Change-Id: Ifce6f505b9e7ba3717bad8f454077a2e94ea6e75
Reviewed-on: https://go-review.googlesource.com/c/146343Reviewed-by: Ian Lance Taylor <iant@golang.org>