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.go b/src/pkg/syscall/syscall_openbsd.go
index 89c7321..a5b87ab 100644
--- a/src/pkg/syscall/syscall_openbsd.go
+++ b/src/pkg/syscall/syscall_openbsd.go
@@ -90,6 +90,11 @@
 	return
 }
 
+//sys getdents(fd int, buf []byte) (n int, err error)
+func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
+	return getdents(fd, buf)
+}
+
 // TODO
 func sendfile(outfd int, infd int, offset *int64, count int) (written int, err error) {
 	return -1, ENOSYS
@@ -119,7 +124,6 @@
 //sys	Fstatfs(fd int, stat *Statfs_t) (err error)
 //sys	Fsync(fd int) (err error)
 //sys	Ftruncate(fd int, length int64) (err error)
-//sys	Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error)
 //sysnb	Getegid() (egid int)
 //sysnb	Geteuid() (uid int)
 //sys	Getfsstat(buf []Statfs_t, flags int) (n int, err error)