rpcrt4: Move the setting of retval_ptr outside of any particular stub phase in stub_do_args and stub_do_old_args.

After the change in order of phases it was no longer being set in the 
last phase and so caused all stubless functions to appear to have no 
return value.

Add tests for this.

winedump: Use the DEFINE_THISCALL_WRAPPER macro instead of the GET_THIS macro for functions with __thiscall calling convention.

Wine doesn't use GET_THIS but DEFINE_THISCALL_WRAPPER.

The header file generated with "winedump spec" without the "-c" option
just includes other headers.