thiscall on MSVC

Hello Benjamin,

Is there a workaround for this? Or how hard would it be to add a separate
"thiscall" convention which maps to "ccc" on GCC-like targets and the custom
MSVC target on MSVC-x86?

I don't think it's good idea to have calling convention to be lowered
differently depending on subtarget: everything should be modelled

It isn't hard to add this CC, because it's just slight variation of
fastcall CC.