Official freeRam() command

All mcu project may pay attention on Ram use, and have an official command for print freeRam at runtime it is very important.
For avr there is a well know freeram function, but it do not work on other architecture, like Zero, Due, etc.

So i think that an official freeRam() function is actually very needed inside the Arduino core, and this also would push third-party core authors to work on their implementation of freeRam().

该 issue 未指定文件、测试或 API 入口点。首先比较现有的 AVR freeRam 方法与此处所述的 Zero 和 Due 架构问题；当形成一个面向整个 core 的、达成共识的 freeRam() API，并且受影响的各架构都有受支持的实现时，即视为完成。

由索引模型根据 Issue 内容生成。