gdb: initialise extension languages after processing early startup files

Now (thanks to the last few commits) all extension languages are
fully initialised in their finish_initialization method, this commit
delays the call to this method until after the early initialization
files have been processed.

Right now there's no benefit from doing this, but in a later commit I
plan to add new options for Python that will control how Python is
initialized.

With this commit in place, my next commits will allow the user to add
options to their early initialization file and alter how Python starts
up.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* main.c (captured_main_1): Add a call to
	finish_ext_lang_initialization.
	* top.c (gdb_init): Remove call to finish_ext_lang_initialization.