Switch nanotime to a monotonic clock on openbsd/386 and openbsd/amd64.
Also use a monotonic clock when for thrsleep, since the sleep duration
is based on the value returned from nanotime.

Update #6007

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/68460044