unix: add gccgo support

With these changes the package builds, and the tests pass, using gccgo
on amd64 GNU/Linux.  I have not tested other systems but I don't know
why they wouldn't work.

Change-Id: I727365daef55f158657eb89afbfcdbf3334610d1
Reviewed-on: https://go-review.googlesource.com/9842
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

unix/gccgo_linux_amd64.go

@@ -0,0 +1,20 @@
+// Copyright 2015 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 gccgo,linux,amd64
+
+package unix
+
+import "syscall"
+
+//extern gettimeofday
+func realGettimeofday(*Timeval, *byte) int32
+
+func gettimeofday(tv *Timeval) (err syscall.Errno) {
+	r := realGettimeofday(tv, nil)
+	if r < 0 {
+		return syscall.GetErrno()
+	}
+	return 0
+}