Treat classes as structs in the amd64 ABI.
Treat classes as structs in the amd64 ABI.

Technically, classes must be treated differently if they have "either a
non-trivial copy constructor or a non-trivial destructor", but since there
is no frontend with these features, there is no harm in treating classes
and structs equally.

MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,libfirm/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm