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.
The test case testsuite/libffi.call/many.c can spuriously fail due to
excess floating-point precision. Instrumenting it with some printf
calls shows differences well above FLT_EPSILON. (Note when
instrumenting it that multiple computations of the difference, such as
one in a print and another in the conditional, may produce different
results.)
Rather than complicating the test suite with architecture-specific flags
to avoid excess precision, just simplify the floating-point computation
to avoid a dependency on potential excess precision.
