Handle classes with a huge number of direct methods.

Make sure that fields Class::copied_methods_offset_ and
Class::virtual_method_offset_ are correctly interpreted as unsigned
16-bit integers.

Test: art/test/testrunner/testrunner.py -j4 -t 648-many-direct-methods
Bug: 33650497
Change-Id: I63e97d5b7e08d58252ff4831b63c4035ecc55979