runtime: skip sysmon workaround on NetBSD >= 9.2

Detect the NetBSD version in osinit and only enable the workaround for
the kernel bug identified in #42515 for NetBSD versions older than 9.2.

For #42515
For #46495

Change-Id: I808846c7f8e47e5f7cc0a2f869246f4bd90d8e22
Reviewed-on: https://go-review.googlesource.com/c/go/+/324472


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
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