This eliminates all the *_win32.c tests in favor of the tests they were
branched from, and expands test coverage to run many more tests on
stdcall, thiscall, and fastcall.
This same mechanism also supports testing any other target that has
multiple ABIs.
This commons up code from libffi.call/call.exp and
libffi.special/special.exp, unifies the optimization option matrix
between the two, and makes it easier to add more axes to the matrix
in the future.
