unix: add utimensat and use it for UtimesNano on BSD

Follow golang.org/cl/55130 and add the utimensat syscall on *BSD. Use it
in UtimesNano and UtimesNanoAt (which is added on *BSD, akin to the
already existing implementation on Linux and Solaris).

Also add AT_FDCWD and AT_SYMLINK_NOFOLLOW where they are missing. These
might be used with UtimesNanoAt.

In order to be able to generate the syscalls, also add two missing
$GOOS_$GOARCH patterns to mkall.sh. As a side effect, some additional
syscalls are added for openbsd/arm.

Change-Id: I85351098002209f8454ec328cef0cfe9d12c5214
Reviewed-on: https://go-review.googlesource.com/55071
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/unix/zsyscall_dragonfly_amd64.go b/unix/zsyscall_dragonfly_amd64.go
index eafceb8..6322630 100644
--- a/unix/zsyscall_dragonfly_amd64.go
+++ b/unix/zsyscall_dragonfly_amd64.go
@@ -1391,3 +1391,18 @@
 	}
 	return
 }
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
+func utimensat(dirfd int, path string, times *[2]Timespec, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_UTIMENSAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(times)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}