gdb/solib: make implementation of solib_ops::open_symbol_file_object optional

The only solib implementation that implements open_symbol_file_object is
SVR4.  All others just return 0.  Make it optional, to avoid having
these empty functions.

Change-Id: I835197a73323d39231d071f9a9eaac2553f10726
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>
6 files changed