Porting to EFM32

Actually, some code in Espruino checks where the stack is vs the end of memory, so it tries not to overflow the stack by recursing too far. It could be that you just need to define those variables in the linker file at the right place. In fact I think build_platform_config actually lets you change the name of them (or some of them) if you don't want to change the linker file: https://github.com/espruino/Espruino/blob/master/scripts/build_platform_config.py#L161

For _sbrk, often what I do in these cases is define the symbol in a C file, compile it, and the search the assembly code for _sbrk in order to see what is using it (it might be something got pulled in that shouldn't be)