kinfo_proc: Restore outputting the effective GID

In particular, fixes 'procstat -s' on a live system or a core file (only
if there are less than 16 groups).

Reviewed by: kib
Fixes: be1f7435ef218b1d ("kern: start tracking cr_gid outside of cr_groups[]")
MFC after: 9 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52259

(cherry picked from commit 63a40ca813a9995e8e2dee0142297d9c38106c05)