Use '--readnever' when invoking GDB from gcore.in

Use '--readnever' when invoking GDB from gcore.in

Back when I proposed the '--readnever' feature, I somehow forgot or
decided not to include the bits related to gcore.in in the original
patch.  This patch finally updates the gcore script to invoke GDB
using '--readnever'.

We've been carrying this patch on Fedora GDB for quite some time, and
as expected the corefiles generated by gcore on Fedora don't have
problems, which I think is the best indicator that the it's safe to
generate corefiles using '--readnever'.

gdb/ChangeLog:
2019-02-23  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gcore.in: Add '--readnever' option when invoking GDB.