os/user: allow build on aix/ppc64 with cgo

This commit adds C support for os/user package for aix/ppc64.

Updates #30563

Change-Id: Id07646998a7243b1335b85b5d4fe5bc4114e2a88
Reviewed-on: https://go-review.googlesource.com/c/go/+/164039
Run-TryBot: Mikio Hara <mikioh.public.networking@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/user/listgroups_aix.go

+// Copyright 2019 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 cgo,!osusergo
+
+package user
+
+import "fmt"
+
+func listGroups(u *User) ([]string, error) {
+	return nil, fmt.Errorf("user: list groups for %s: not supported on AIX", u.Username)
+}

src/os/user/user_test.go

@@ -129,6 +129,9 @@ func TestLookupGroup(t *testing.T) {
 
 func TestGroupIds(t *testing.T) {
 	checkGroup(t)
+	if runtime.GOOS == "aix" {
+		t.Skip("skipping GroupIds, see golang.org/issue/30563")
+	}
 	if runtime.GOOS == "solaris" {
 		t.Skip("skipping GroupIds, see golang.org/issue/14709")
 	}