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