test/plain_c++: - refactored to share code - extended to treat default this calls and explicitly as cdecl declared ones, separately - prev point fixes implicitly on x86 a wrong dcMode setting (assumed that cdecl for methods would default to the native this call convention, but actually does not, applies cdecl verbatim) - extended aggr tests to also test explicit cdecl as well as MS thiscalls on x86 - simplified