Fix build with Python 3.4 (PR python/16784)

The type of the function pointer PyOS_ReadlineFunctionPointer (part of the
Python C API), which we use, slightly changed starting with Python 3.4. The
signature went from

PyAPI_DATA(char) *(*PyOS_ReadlineFunctionPointer)(FILE *, FILE *, char *);

to

PyAPI_DATA(char) *(*PyOS_ReadlineFunctionPointer)(FILE *, FILE *, const char *);

The parameter that changed is the prompt text.

This commits adjust gdb accordingly by making the prompt_arg parameter const,
as well as the fallouts of that. I needed to rework how annotations are added
to the prompt, since the it is now const. If annotations are enabled, it will
make a copy of the prompt and then overwrite prompt_arg to point to the
annotated prompt. Otherwise, no copy is done and the original prompt_arg is
passed.

I changed the signature of deprecated_readline_hook. I would've changed any
user of it, but it seems like nothing is using it,

Built-tested with python 2.7.x, 3.3.y and 3.4.z.

Signed-off-by: Simon Marchi <simon.marchi@ericsson.com>