Fix PR python/17386 - add __index__ method to gdb.Value

Fix PR python/17386 - add __index__ method to gdb.Value

This patch fixes PR python/17386.

The bug is that gdb.Value does not implement the Python __index__
method.  This method is needed to convert a Python object to an index
and is used by various operations in Python, such as indexing an
array.

The fix is to implement the nb_index method for gdb.Value.

nb_index was added in Python 2.5.  I don't have a good way to test
Python 2.4, but I made an attempt to accomodate it.

I chose to use valpy_long in all cases because this simplifies porting
to Python 3, and because there didn't seem to be any harm.

Built and regtested on x86-64 Fedora 23.

2016-05-24  Tom Tromey  <tom@tromey.com>

	PR python/17386:
	* python/py-value.c (value_object_as_number): Add
	nb_inplace_floor_divide, nb_inplace_true_divide, nb_index.

2016-05-24  Tom Tromey  <tom@tromey.com>

	PR python/17386:
	* gdb.python/py-value.exp (test_value_numeric_ops): Add tests that
	use value as an index.