gdb/python: break dependencies between gdbpy_initialize_* functions

In a later commit in this series I will propose removing all of the
explicit gdbpy_initialize_* calls from python.c and replace these
calls with a more generic mechanism.

One of the side effects of this generic mechanism is that the order in
which the various Python sub-systems within GDB are initialized is no
longer guaranteed.

On the whole I don't think this matters, most of the sub-systems are
independent of each other, though testing did reveal a few places
where we did have dependencies, though I don't think those
dependencies were explicitly documented in a comment anywhere.

This commit removes the first dependency issue, with this and the next
commit, all of the implicit inter-sub-system dependencies will be
replaced by explicit dependencies, which will allow me to, I think,
clean up how the sub-systems are initialized.

The dependency is around the py_insn_type.  This type is setup in
gdbpy_initialize_instruction and used in gdbpy_initialize_record.
Rather than depend on the calls to these two functions being in a
particular order, in this commit I propose adding a new function
py_insn_get_insn_type.  This function will take care of setting up the
py_insn_type type and calling PyType_Ready.  This helper function can
be called from gdbpy_initialize_record and
gdbpy_initialize_instruction, and the py_insn_type will be initialized
just once.

To me this is better, the dependency is now really obvious, but also,
we no longer care in which order gdbpy_initialize_record and
gdbpy_initialize_instruction are called.

There should be no user visible changes after this commit.

gdb/python/py-record.c

@@ -563,7 +563,7 @@ gdbpy_initialize_record (void)
   recpy_insn_type.tp_getset = recpy_insn_getset;
   recpy_insn_type.tp_richcompare = recpy_element_richcompare;
   recpy_insn_type.tp_hash = recpy_element_hash;
-  recpy_insn_type.tp_base = &py_insn_type;
+  recpy_insn_type.tp_base = py_insn_get_insn_type ();
 
   recpy_func_type.tp_new = PyType_GenericNew;
   recpy_func_type.tp_flags = Py_TPFLAGS_DEFAULT;