compiler: Fix backend representation of calls to interface methods.

Also unify all identical result parameter sets into a single
struct type, and fix the use of backend function pointers.

	* go-gcc.cc (Gcc_backend::function_type): Add result_struct
	parameter.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@205316 138bc75d-0d04-0410-961f-82ee72b054a4