Fix redefinition errors in C++ mode

In C, we can forward declare static structure instances.  That doesn't
work in C++ though.  C++ treats these as definitions.  So then the
compiler complains about symbol redefinition, like:

 src/gdb/elfread.c:1569:29: error: redefinition of ‘const sym_fns elf_sym_fns_lazy_psyms’
 src/gdb/elfread.c:53:29: error: ‘const sym_fns elf_sym_fns_lazy_psyms’ previously declared here

The intent of static here is naturally to avoid making these objects
visible outside the compilation unit.  The equivalent in C++ would be
to instead define the objects in the anonymous namespace.  But given
that it's desirable to leave the codebase compiling as both C and C++
for a while, this just makes the objects extern.

(base_breakpoint_ops is already declared in breakpoint.h, so we can
just remove the forward declare from breakpoint.c)

gdb/ChangeLog:
2015-02-11  Tom Tromey  <tromey@redhat.com>
	    Pedro Alves <palves@redhat.com>

	* breakpoint.c (base_breakpoint_ops): Delete.
	* dwarf2loc.c (dwarf_expr_ctx_funcs): Make extern.
	* elfread.c (elf_sym_fns_gdb_index, elf_sym_fns_lazy_psyms): Make extern.
	* guile/guile.c (guile_extension_script_ops, guile_extension_ops): Make extern.
	* ppcnbsd-tdep.c (ppcnbsd2_sigtramp): Make extern.
	* python/py-arch.c (arch_object_type): Make extern.
	* python/py-block.c (block_syms_iterator_object_type): Make extern.
	* python/py-bpevent.c (breakpoint_event_object_type): Make extern.
	* python/py-cmd.c (cmdpy_object_type): Make extern.
	* python/py-continueevent.c (continue_event_object_type)
	* python/py-event.h (GDBPY_NEW_EVENT_TYPE): Remove 'qual'
	parameter.  Update all callers.
	* python/py-evtregistry.c (eventregistry_object_type): Make extern.
	* python/py-exitedevent.c (exited_event_object_type): Make extern.
	* python/py-finishbreakpoint.c (finish_breakpoint_object_type): Make extern.
	* python/py-function.c (fnpy_object_type): Make extern.
	* python/py-inferior.c (inferior_object_type, membuf_object_type): Make extern.
	* python/py-infevents.c (call_pre_event_object_type)
	(inferior_call_post_event_object_type).
	(memory_changed_event_object_type): Make extern.
	* python/py-infthread.c (thread_object_type): Make extern.
	* python/py-lazy-string.c (lazy_string_object_type): Make extern.
	* python/py-linetable.c (linetable_entry_object_type)
	(linetable_object_type, ltpy_iterator_object_type): Make extern.
	* python/py-newobjfileevent.c (new_objfile_event_object_type)
	(clear_objfiles_event_object_type): Make extern.
	* python/py-objfile.c (objfile_object_type): Make extern.
	* python/py-param.c (parmpy_object_type): Make extern.
	* python/py-progspace.c (pspace_object_type): Make extern.
	* python/py-signalevent.c (signal_event_object_type): Make extern.
	* python/py-symtab.c (symtab_object_type, sal_object_type): Make extern.
	* python/py-type.c (type_object_type, field_object_type)
	(type_iterator_object_type): Make extern.
	* python/python.c (python_extension_script_ops)
	(python_extension_ops): Make extern.
	* stap-probe.c (stap_probe_ops): Make extern.