compiler, runtime: Use runtime functions to pass closure value.

compiler, runtime: Use runtime functions to pass closure value.

This changes the compiler and runtime to not pass a closure
value as the last argument, but to instead pass it via
__go_set_closure and retrieve it via __go_get_closure.  This
eliminates the need for function descriptor wrapper functions.
It will make it possible to retrieve the closure value in a
reflect.MakeFunc function.

From-SVN: r202233