runtime: fix erroneous overflow protection on netbsd/openbsd semasleep.

On NetBSD tv_sec is already an int64 so no need for a test.

On OpenBSD, semasleep expects a Unix time as argument,
and 1<<30 is in 2004.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7810044