compiler, reflect, runtime: Use static chain for closures.

compiler, reflect, runtime: Use static chain for closures.

Change from using __go_set_closure to passing the closure
value in the static chain field.  Uses new backend support for
setting the closure chain in a call from C via
__builtin_call_with_static_chain.  Uses new support in libffi
for Go closures.

The old architecture specific support for reflect.MakeFunc is
removed, replaced by the libffi support.

All work done by Richard Henderson.

	* go-gcc.cc (Gcc_backend::call_expression): Add chain_expr argument.
	(Gcc_backend::static_chain_variable): New method.

From-SVN: r219776