Add extended segment support to gcore

A follow-up to r303099, D7255. Basically, apply the exact same change, with
the exact same rationale, to gcore. gcore's elfcore.c is largely a clone of
the kernel imgact_elf coredump facility.

Reviewed by: emaste (earlier version, not substantially different)
Requested by: jhb
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D7265