runtime: detect netbsd netpoll overrun in sysmon

The netbsd kernel has a bug [1] that occassionally prevents netpoll from
waking with netpollBreak, which could result in missing timers for an
unbounded amount of time, as netpoll can't restart with a shorter delay
when an earlier timer is added.

Prior to CL 232298, sysmon could detect these overrun timers and
manually start an M to run them. With this fallback gone, the bug
actually prevents timer execution indefinitely.

As a workaround, we add back sysmon detection only for netbsd.

[1] https://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=50094

Updates #42515

Change-Id: I8391f5b9dabef03dd1d94c50b3b4b3bd4f889e66
Reviewed-on: https://go-review.googlesource.com/c/go/+/277332


Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Trust: Michael Pratt <mpratt@google.com>