compiler: Use backend interface for stack allocation.

compiler: Use backend interface for stack allocation.

Stack allocation was being done by making a temporary variable and
taking its address.  This does not work when allocating in a loop
because every allocated variable will refer to the same address.
The backend now provides a way to safely allocate in a loop.

	* go-gcc.cc (Gcc_backend::stack_allocation_expression): New
	method.

From-SVN: r222657