Add a '-l' flag to testgldispatch, which tells it to test a generated stub
at the very end of the dispatch table.
On PPC, looking up a function past index 4096 requires more than just a simple
register+immediate load, so this makes sure that it's correct.
Replaced testpatchentrypoints_gldispatch with three new unit tests.
testgldispatch_static and testgldispatch_generated test dispatching to multiple
vendor libraries through static and generated dispatch stubs, respectively.
testgldispatch_generated tests entrypoint patching and unpatching. This adds
coverage for patching generated stubs in addition to the static ones.
The entrypoint patching functions in tests/dummy/patchentrypoints.c now take
the name of the function to patch, instead of hard-coding it to only patch
glVertex3fv.
