runtime, syscall: update for openbsd system ABI break

Update Go so that it continues to work past the OpenBSD system ABI
break, with 64-bit time_t:

  http://www.openbsd.org/faq/current.html#20130813

Note: this makes OpenBSD 5.5 (currently 5.4-current) the minimum
supported release for Go.

Fixes #7049.

R=golang-codereviews, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/13368046
diff --git a/src/pkg/syscall/syscall_openbsd_386.go b/src/pkg/syscall/syscall_openbsd_386.go
index 3c4c693..ad5ae14 100644
--- a/src/pkg/syscall/syscall_openbsd_386.go
+++ b/src/pkg/syscall/syscall_openbsd_386.go
@@ -9,7 +9,7 @@
 func TimespecToNsec(ts Timespec) int64 { return int64(ts.Sec)*1e9 + int64(ts.Nsec) }
 
 func NsecToTimespec(nsec int64) (ts Timespec) {
-	ts.Sec = int32(nsec / 1e9)
+	ts.Sec = int64(nsec / 1e9)
 	ts.Nsec = int32(nsec % 1e9)
 	return
 }
@@ -19,7 +19,7 @@
 func NsecToTimeval(nsec int64) (tv Timeval) {
 	nsec += 999 // round up to microsecond
 	tv.Usec = int32(nsec % 1e9 / 1e3)
-	tv.Sec = int32(nsec / 1e9)
+	tv.Sec = int64(nsec / 1e9)
 	return
 }