runtime: fix semasleep() duration for openbsd
runtime: fix semasleep() duration for openbsd

The timespec passed to thrsleep() needs to be an absolute/realtime
value, so add the current nanotime to ns.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5374048

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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