libprocstat: procstat_getgroups_kvm(): Output again the effective GID

In particular, fixes 'procstat -s -M' (for processes with more than 16
groups).

Reviewed by: kib
Fixes: be1f7435ef218b1d ("kern: start tracking cr_gid outside of cr_groups[]")
MFC after: 5 days
MFC to: stable/15
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52275