internal/syscall/unix: add constants for hurd

Patch by Svante Signell.
    
    Reviewed-on: https://go-review.googlesource.com/c/161517

From-SVN: r268602

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-d89db31db68d09aa13a6137122cc096c1d92597b
+77f0f28af556f50c561ff5c2cca17ad6f985068e
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/internal/syscall/unix/at_sysnum_hurd.go

@@ -0,0 +1,8 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package unix
+
+const AT_REMOVEDIR = 0x200
+const AT_SYMLINK_NOFOLLOW = 0x100