Prevent malloc from overwriting the stack

[matthijskooijman]

The current implementation of malloc (or really, the underlying sbrk() implementation) happily allocates any amount of memory, even when that overwrites the stack or even beyond the end of RAM. This is fixed by implementing a custom sbrk() function that does proper checking. The code is based on the sbrk() from the STM32 Arduino core, but with an additional margin added (to make it fail when it comes close to the stack, instead of just when it would actually overwrite the stack).

This margin approach is copied from avr-libc's malloc implementation. I considered also copying more of avr-libc's configurability (e.g. __malloc_heap_start and __malloc_heap_end), but that ended up just adding complexity without a very clear usecase, so I left that out.

See the d232b59 commit message for much more detail on this problem and the solution.

This PR also has a somewhat unrelated commit removing the -nostdlib compilation option, which was in the wrong place and thus effectively unused (and also unneeded).