unix: fix definition of AT_FDCWD on dragonfly

Follow CL golang.org/cl/55690 (which did the same for syscall) and fix
the definition of AT_FDCWD on dragonfly/amd64.

Change-Id: I2c44a0f84308697d3dbac9b78c4a3dce973f88ba
Reviewed-on: https://go-review.googlesource.com/55691
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/ztypes_dragonfly_amd64.go

@@ -443,6 +443,6 @@ type Termios struct {
 }
 
 const (
-	AT_FDCWD            = -0x64
+	AT_FDCWD            = 0xfffafdcd
 	AT_SYMLINK_NOFOLLOW = 0x1
 )