unix: move gccgo redeclared *SyscallNoError functions to a separate file

Same as CL 34379, SyscallNoError and RawSyscallNoError are redeclared
for gccgo builds since CL 85756. Thus, move the declarations to a gc
specific file.

Updates golang/go#18312

Change-Id: Ia8f42b31e61a80530e5e90a44c0af32fd3452612
Reviewed-on: https://go-review.googlesource.com/91615
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux.go

@@ -16,13 +16,6 @@ import (
 	"unsafe"
 )
 
-// SyscallNoError may be used instead of Syscall for syscalls that don't fail.
-func SyscallNoError(trap, a1, a2, a3 uintptr) (r1, r2 uintptr)
-
-// RawSyscallNoError may be used instead of RawSyscall for syscalls that don't
-// fail.
-func RawSyscallNoError(trap, a1, a2, a3 uintptr) (r1, r2 uintptr)
-
 /*
  * Wrapped
  */

unix/syscall_linux_gc.go

+// Copyright 2018 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.
+
+// +build linux,!gccgo
+
+package unix
+
+// SyscallNoError may be used instead of Syscall for syscalls that don't fail.
+func SyscallNoError(trap, a1, a2, a3 uintptr) (r1, r2 uintptr)
+
+// RawSyscallNoError may be used instead of RawSyscall for syscalls that don't
+// fail.
+func RawSyscallNoError(trap, a1, a2, a3 uintptr) (r1, r2 uintptr)