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_amd64.go b/src/pkg/syscall/syscall_openbsd_amd64.go
index c356ad4..6181344 100644
--- a/src/pkg/syscall/syscall_openbsd_amd64.go
+++ b/src/pkg/syscall/syscall_openbsd_amd64.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 = nsec / 1e9
 	ts.Nsec = nsec % 1e9
 	return
 }
@@ -19,12 +19,12 @@
 func NsecToTimeval(nsec int64) (tv Timeval) {
 	nsec += 999 // round up to microsecond
 	tv.Usec = nsec % 1e9 / 1e3
-	tv.Sec = int64(nsec / 1e9)
+	tv.Sec = nsec / 1e9
 	return
 }
 
 func SetKevent(k *Kevent_t, fd, mode, flags int) {
-	k.Ident = uint32(fd)
+	k.Ident = uint64(fd)
 	k.Filter = int16(mode)
 	k.Flags = uint16(flags)
 }