syscall: don't define syscall stub on Hurd

Patch from Samuel Thibault.

Change-Id: I782d6c702c086352a61df1574747bab0745ff545
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/623415
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

libgo/go/syscall/syscall_funcs.go

diff --git a/libgo/go/syscall/syscall_funcs.go b/libgo/go/syscall/syscall_funcs.go
index a906fa5..fc14cb1 100644
--- a/libgo/go/syscall/syscall_funcs.go
+++ b/libgo/go/syscall/syscall_funcs.go
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build darwin || dragonfly || freebsd || hurd || linux || netbsd || openbsd || solaris
-// +build darwin dragonfly freebsd hurd linux netbsd openbsd solaris
+//go:build darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
 
 package syscall
 

libgo/go/syscall/syscall_funcs_stubs.go

diff --git a/libgo/go/syscall/syscall_funcs_stubs.go b/libgo/go/syscall/syscall_funcs_stubs.go
index 11f12bd..e37a648 100644
--- a/libgo/go/syscall/syscall_funcs_stubs.go
+++ b/libgo/go/syscall/syscall_funcs_stubs.go
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build aix || rtems
-// +build aix rtems
+//go:build aix || hurd || rtems
+// +build aix hurd rtems
 
 // These are stubs.
 

libgo/runtime/go-nosys.c

diff --git a/libgo/runtime/go-nosys.c b/libgo/runtime/go-nosys.c
index 30222df..cd3e766 100644
--- a/libgo/runtime/go-nosys.c
+++ b/libgo/runtime/go-nosys.c
@@ -504,7 +504,7 @@
 
 #endif /* ! HAVE_STRERROR_R */
 
-#ifndef HAVE_SYSCALL
+#if !defined(HAVE_SYSCALL) && !defined(__GNU__) /* GNU/Hurd already has a stub */
 int
 syscall(int number __attribute__ ((unused)), ...)
 {