kvm_proclist(): Restore outputting the effective GID

In particular, fixes 'procstat -s -M' (only if there are less than 16
groups).

Reviewed by: kib, emaste
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/D52260

(cherry picked from commit 7676df2faeb6dcbf20456574dcd2a7f4ab8fff8a)