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>