amd64: Fix segfault for functions with variable arguments

(printf now works with floating point)

ir/be/amd64/amd64_cconv.c

@@ -198,6 +198,11 @@ amd64_cconv_t *amd64_decide_calling_convention(ir_type *function_type,
 		}
 	}
 
+	/* vararg calls need additional stack space */
+	if (get_method_variadicity(function_type) == variadicity_variadic) {
+		stack_offset += AMD64_REGISTER_SIZE;
+	}
+
 	amd64_cconv_t *cconv    = XMALLOCZ(amd64_cconv_t);
 	cconv->parameters       = params;
 	cconv->param_stack_size = stack_offset;