they have two RAMs at two distinct locations:

RAM1 (address: MBED_RAM_START, size: MBED_RAM_SIZE):
* stack
* heap
* some part of static memory

RAM2 (address: MBED_IRAM2_START, size: MBED_IRAM2_SIZE):
* remaining part of static memory starting at MBED_IRAM2_START
* crash report
* vector

The heap size was incorrectly calculated.
This fixes it by subtracting the Stack size, any memory chunks allocated
before the start of the application (for vectors and/or crash report), and
finally the size of the application from the total RAM size.

This is required for ARMClang, otherwise there is an error with unknown command.

- Alignment with other STM32 families