When building against Python 2.7, -Wwrite-strings flags several cases
of passing a string literal to Python functions that expect a "char
*".  This commit addresses the issue like we already handle several
other similar cases -- wrap the Python API with our own fixed
version that adds the necessary constification.

gdb/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* python/python-internal.h (gdb_PyObject_CallMethod)
	(gdb_PyErr_NewException, gdb_PySys_GetObject, gdb_PySys_SetPath):
	New functions.
	(GDB_PYSYS_SETPATH_CHAR, PyObject_CallMethod, PyErr_NewException)
	(PySys_GetObject, PySys_SetPath): New macros.