Remove unused heap from startup assembly

[forGGe]

Startup code contains heap reservation:

https://github.com/forGGe/theCore/blob/c81a18a1d7e8ead263625c4fab9904fe2a50a9a4/arch/arm_cm/startup_arm_cm.S#L56-L60

However, there is no need for heap in many cases, so if heap size is not set, heap should be eliminated. Implementation must be re-tested on all embedded platforms to make sure heap is not used there.

[forGGe]

Refs: https://stackoverflow.com/questions/40130374/gcc-how-to-disable-heap-usage-entirely-on-an-mcu