Convert CORE_ADDR to Python using gdb_py_object_from_ulongest

Convert CORE_ADDR to Python using gdb_py_object_from_ulongest

An internal test failed on a riscv64-elf cross build because
Inferior.search_memory returned a negative value.  I tracked this down
to to use of PyLong_FromLong in infpy_search_memory.  Then, I looked
at other conversions of CORE_ADDR to Python and fixed these as well.

I don't think there is a good way to write a test for this.

gdb/ChangeLog
2020-08-17  Tom Tromey  <tromey@adacore.com>

	* python/py-inferior.c (infpy_search_memory): Use
	gdb_py_object_from_ulongest.
	* python/py-infevents.c (create_inferior_call_event_object)
	(create_memory_changed_event_object): Use
	gdb_py_object_from_ulongest.
	* python/py-linetable.c (ltpy_entry_get_pc): Use
	gdb_py_object_from_ulongest.