x/sys/unix: move gccgo redeclared functions to separate files

These functions are redeclared in gccgo builds, so move them
to separate files with the "!gccgo" build constraint.

Fixes golang/go#18312

Change-Id: I868cfb77fc394e5e96cee78af0f3abf43db6082a
Reviewed-on: https://go-review.googlesource.com/34379
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux_amd64_gc.go

@@ -0,0 +1,13 @@
+// Copyright 2016 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 amd64,linux
+// +build !gccgo
+
+package unix
+
+import "syscall"
+
+//go:noescape
+func gettimeofday(tv *Timeval) (err syscall.Errno)