Wasm backend doesn't handle ForeignHints

When the wasm NCG lowers a ccall, the ForeignHints of arguments and return value is not handled. This is fine for W32/W64, but in case of W8/W16, it leads to incorrect runtime behavior. For instance:

           _s11Q::P32 = R1;
           _s11R::I16 = I16[_s11Q::P32 + 3];
           _c12t::I32 = func16;
           _c12u::I16 = _s11R::I16;
           (_s11V::I16) = call "ccall" arg hints:  [‘signed’]  result hints:  [‘signed’] (_c12t::I32)(_c12u::I16);

Due to Cmm not tracking signedness at type-level, when we load _s11R, we always have to do i32.load16_u. Now, when it's passed as a ccall argument, given the signed hint, we must do an explicit i32.extend16_s first. And we didn't do that. Sigh. Leads to T20735 failure and possibly more.