Even with -D_POSIX_PTHREAD_SEMANTICS, Solaris seems to not define
getgrnam_r in a POSIX compatible way.

Fixes #14967

Change-Id: I78cb7e5b30b2d8b860e336060a0a06f4720c0475
Reviewed-on: https://go-review.googlesource.com/21385Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>