os/user: use getgrouplist on illumos && cgo

The getgrouplist call is available on Illumos since December 2020:

    illumos/illumos-gate@f2c438c

We can assume it is available for users now. Let's switch to using it
when cgo is enabled.

Since neither LUCY nor legacy trybots provide illumos, I tested this
locally in a OpenIndiana VM, with and without osusergo, with cgo enabled
and disabled.

This is a continuation of CL 315278.

Fixes #14709

Change-Id: I922049e7ea5f450f6900914b30967e522e56cfc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/702975
Reviewed-by: Andrew Stormont <[email protected]>
Auto-Submit: Damien Neil <[email protected]>
Reviewed-by: Tobias Klauser <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Auto-Submit: Kirill Kolyshkin <[email protected]>
TryBot-Bypass: Kirill Kolyshkin <[email protected]>

Author: kolyshkin

src/os/user/cgo_listgroups_unix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (cgo || darwin) && !osusergo && (darwin || dragonfly || freebsd || (linux && !android) || netbsd || openbsd || (solaris && !illumos))
+//go:build (cgo || darwin) && !osusergo && (darwin || dragonfly || freebsd || (linux && !android) || netbsd || openbsd || solaris)
 
 package user
 

src/os/user/getgrouplist_unix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build cgo && !osusergo && (dragonfly || freebsd || (!android && linux) || netbsd || openbsd || (solaris && !illumos))
+//go:build cgo && !osusergo && (dragonfly || freebsd || (!android && linux) || netbsd || openbsd || solaris)
 
 package user
 

src/os/user/listgroups_unix.go

@@ -2,7 +2,7 @@
 // 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 || (js && wasm) || wasip1 || (!android && linux) || netbsd || openbsd || solaris) && ((!cgo && !darwin) || osusergo)) || aix || illumos
+//go:build ((darwin || dragonfly || freebsd || (js && wasm) || wasip1 || (!android && linux) || netbsd || openbsd || solaris) && ((!cgo && !darwin) || osusergo)) || aix
 
 package user
 

src/os/user/listgroups_unix_test.go

@@ -2,7 +2,7 @@
 // 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 || (js && wasm) || wasip1 || (!android && linux) || netbsd || openbsd || solaris) && ((!cgo && !darwin) || osusergo)) || aix || illumos
+//go:build ((darwin || dragonfly || freebsd || (js && wasm) || wasip1 || (!android && linux) || netbsd || openbsd || solaris) && ((!cgo && !darwin) || osusergo)) || aix
 
 package user