[LPC4088] RAM size fix for ARM_GCC. Added possibility to reserve stack space

Since the internal RAM by default is used both for heap and stack space it is possible for the heap and stack to collide if a lot of heap allocations are made (e.g by a graphical application). Adding the possibility to reserve stack space solves this problem and is typically used in combination with support for external RAM (SDRAM.cpp in EALib)