amd64_cconv: Always reserve a multiple of 8 bytes on the stack.

This is relevant for long doubles, which are 80 bits long, but take up
128 bits on the stack and in memory according to the AMD64 ABI.

ir/be/amd64/amd64_cconv.c

@@ -128,7 +128,7 @@ x86_cconv_t *amd64_decide_calling_convention(ir_type *function_type,
 			param->offset = stack_offset;
 			/* increase offset by at least AMD64_REGISTER_SIZE bytes so
 			 * everything is aligned */
-			stack_offset += MAX(bits / 8, AMD64_REGISTER_SIZE);
+			stack_offset += round_up2(bits / 8, AMD64_REGISTER_SIZE);
 		}
 	}