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://golang.org/cl/7810044

src/pkg/runtime/os_netbsd.c

@@ -65,7 +65,6 @@ int32
 runtime·semasleep(int64 ns)
 {
 	Timespec ts;
-	int64 secs;
 
 	// spin-mutex lock
 	while(runtime·xchg(&m->waitsemalock, 1))
@@ -94,11 +93,7 @@ runtime·semasleep(int64 ns)
 				runtime·lwp_park(nil, 0, &m->waitsemacount, nil);
 			} else {
 				ns += runtime·nanotime();
-				secs = ns/1000000000LL;
-				// Avoid overflow
-				if(secs > 1LL<<30)
-					secs = 1LL<<30;
-				ts.tv_sec = secs;
+				ts.tv_sec = ns/1000000000LL;
 				ts.tv_nsec = ns%1000000000LL;
 				// TODO(jsing) - potential deadlock!
 				// See above for details.

src/pkg/runtime/os_openbsd.c

@@ -79,8 +79,8 @@ runtime·semasleep(int64 ns)
 				ns += runtime·nanotime();
 				secs = ns/1000000000LL;
 				// Avoid overflow
-				if(secs > 1LL<<30)
-					secs = 1LL<<30;
+				if(secs >= 1LL<<31)
+					secs = (1LL<<31) - 1;
 				ts.tv_sec = secs;
 				ts.tv_nsec = ns%1000000000LL;
 				runtime·thrsleep(&m->waitsemacount, CLOCK_REALTIME, &ts, &m->waitsemalock, nil);