os/user: add AIX getgroups support

[Helflym]

getgroups isn't yet available on AIX, as getgrouplist syscall doesn't exist on AIX.
Therefore, https://go-review.googlesource.com/c/go/+/164039 disables tests.

This is a tracking-bug.

[gopherbot]

Change https://golang.org/cl/164039 mentions this issue: os/user: allow build on aix/ppc64 with cgo

[gopherbot]

Change https://golang.org/cl/330753 mentions this issue: os/user: implement go native GroupIds

[kolyshkin]

So, AIX does not have getgrouplist but it does have getgrset and getgrset_r. Alas I don't have any experience with AIX but it seems it might be possible to use it if cgo is enabled.