runtime: cleanup openbsd semasleep implementation
The previous implementation had several subtle issues. It's not clear if any of these could actually be causing the flakiness problems on openbsd/386, but fixing them should only help. 1. thrsleep() is implemented internally as unlock, then test *abort (if abort != nil), then tsleep(). Under the current code, that makes it theoretically possible that semasleep()/thrsleep() could release waitsemalock, then a racing semawakeup() could acquire the lock, increment waitsemacount, and call thrwakeup()/wakeup() before thrsleep() reaches tsleep(). (In practice, OpenBSD's big kernel lock seems unlikely to let this actually happen.) The proper way to avoid this is to pass &waitsemacount as the abort pointer to thrsleep so thrsleep knows to re-check it before going to sleep, and to wakeup if it's non-zero. Then we avoid any races. (I actually suspect openbsd's sema{sleep,wakeup}() could be further simplified using cas/xadd instead of locks, but I don't want to be more intrusive than necessary so late in the 1.4 release cycle.) 2. semasleep() takes a relative sleep duration, but thrsleep() needs an absolute sleep deadline. Instead of recomputing the deadline each iteration, compute it once up front and use (*Timespec)(nil) to signify no deadline. Ensures we retry properly if there's a spurious wakeup. 3. Instead of assuming if thrsleep() woke up and waitsemacount wasn't available that we must have hit the deadline, check that the system call returned EWOULDBLOCK. 4. Instead of assuming that 64-bit systems are little-endian, compute timediv() using a temporary int32 nsec and then assign it to tv_nsec. LGTM=iant R=jsing, iant CC=golang-codereviews https://golang.org/cl/137960043
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