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().

The issue names no file, test, or API entry point. Start by comparing the existing AVR freeRam approach with the Zero and Due architecture concerns described here; done means an agreed core-wide freeRam() API with implementations supported across the affected architectures.

Written by the indexing model from the issue text.