Fix Python 3 build error on 32-bit hosts

Fix Python 3 build error on 32-bit hosts

on Fedora Rawhide (==22) i686 using --with-python=/usr/bin/python3 one gets:

./python/py-value.c:1696:3: error: initialization from incompatible pointer type [-Werror]
   valpy_hash,            /*tp_hash*/
   ^
./python/py-value.c:1696:3: error: (near initialization for ‘value_object_type.tp_hash’) [-Werror]
cc1: all warnings being treated as errors
Makefile:2628: recipe for target 'py-value.o' failed

This is because in Python 2 tp_hash was:
	typedef long (*hashfunc)(PyObject *);
while in Python 3 tp_hash is:
	typedef Py_hash_t (*hashfunc)(PyObject *);

Py_hash_t is int for 32-bit hosts and long for 64-bit hosts.  While on 32-bit
hosts sizeof(long)==sizeof(int) still the hashfunc type is formally
incompatible.  As this patch should have no compiled code change it is not
really necessary for gdb-7.9, it would fix there just this non-fatal
compilation warning:
	./python/py-value.c:1696:3: warning: initialization from incompatible pointer type
	   valpy_hash,            /*tp_hash*/
	   ^
	./python/py-value.c:1696:3: warning: (near initialization for ‘value_object_type.tp_hash’)

gdb/ChangeLog
2015-02-04  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* python/python-internal.h (Py_hash_t): Define it for Python <3.2.
	* python/py-value.c (valpy_fetch_lazy): Use it.  Remove cast to the
	return type.