syscall: don't define syscall stub on Hurd

Patch from Samuel Thibault.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/623415

Author: ianlancetaylor

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-092668d6ce6d7b3aff6797247cd53dc44319c558
+f9ea9801058aa98a421784da12b76cda0b4c6cf2
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

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

@@ -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

@@ -504,7 +504,7 @@ strerror_r (int errnum, char *buf, size_t buflen)
 
 #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)), ...)
 {