gdb: delay python initialisation until gdbpy_finish_initialization

Delay Python initialisation until gdbpy_finish_initialization.

This is mostly about splitting the existing gdbpy_initialize_*
functions in two, all the calls to register_objfile_data_with_cleanup,
gdbarch_data_register_post_init, etc are moved into new _initialize_*
functions, but everything else is left in the gdbpy_initialize_*
functions.

Then the call to do_start_initialization (in python/python.c) is moved
from the _initialize_python function into gdbpy_finish_initialization.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* python/py-arch.c (_initialize_py_arch): New function.
	(gdbpy_initialize_arch): Move code to _initialize_py_arch.
	* python/py-block.c (_initialize_py_block): New function.
	(gdbpy_initialize_blocks): Move code to _initialize_py_block.
	* python/py-inferior.c (_initialize_py_inferior): New function.
	(gdbpy_initialize_inferior): Move code to _initialize_py_inferior.
	* python/py-objfile.c (_initialize_py_objfile): New function.
	(gdbpy_initialize_objfile): Move code to _initialize_py_objfile.
	* python/py-progspace.c (_initialize_py_progspace): New function.
	(gdbpy_initialize_pspace): Move code to _initialize_py_progspace.
	* python/py-registers.c (_initialize_py_registers): New function.
	(gdbpy_initialize_registers): Move code to
	_initialize_py_registers.
	* python/py-symbol.c (_initialize_py_symbol): New function.
	(gdbpy_initialize_symbols): Move code to _initialize_py_symbol.
	* python/py-symtab.c (_initialize_py_symtab): New function.
	(gdbpy_initialize_symtabs): Move code to _initialize_py_symtab.
	* python/py-type.c (_initialize_py_type): New function.
	(gdbpy_initialize_types): Move code to _initialize_py_type.
	* python/py-unwind.c (_initialize_py_unwind): New function.
	(gdbpy_initialize_unwind): Move code to _initialize_py_unwind.
	* python/python.c (_initialize_python): Move call to
	do_start_initialization to gdbpy_finish_initialization.
	(gdbpy_finish_initialization): Add call to
	do_start_initialization.