syscall: fix definition of _AT_FDCWD on dragonfly

CL golang.org/cl/55130 messed up the definition of _AT_FDCWD on
dragonfly.

This fixes the following test failure on dragonfly/amd64:

  --- FAIL: TestPackageMainTestImportsArchiveNotBinary (0.00s)
  	  go_test.go:192: chtimes ./testdata/src/main_test/m.go: bad file descriptor

Change-Id: I4c96983769e6b02d714859dc838875c3c0f1be50
Reviewed-on: https://go-review.googlesource.com/55690
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/ztypes_dragonfly_amd64.go

@@ -433,7 +433,7 @@ type BpfHdr struct {
 }
 
 const (
-	_AT_FDCWD = -0x64
+	_AT_FDCWD = 0xfffafdcd
 )
 
 type Termios struct {