gdb/solib: add solib -> solib_ops backlink

The subsequent C++ification commit makes it so that one struct solib_ops
is instantiated for each program space.  For some operations, it will
then become necessary to be able to get the right solib_ops instance
from a given solib.  Add an solib -> solib_ops backlink for that.

Change-Id: Ib95407b3fa5fcfba55cf874e0e9dcd2d43a402e4
Approved-By: Pedro Alves <pedro@palves.net>
8 files changed