os/user: use getgrouplist on illumos && cgo
The getgrouplist call is available on Illumos since December 2020:
https://github.com/illumos/illumos-gate/commit/
f2c438c5058c64b7373448f239156bf60009abcb
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 <andyjstormont@gmail.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Kirill Kolyshkin <kolyshkin@gmail.com>
TryBot-Bypass: Kirill Kolyshkin <kolyshkin@gmail.com>