Add abort() implementation

Also reduces the IRAM footprint of the fatal exception handler, as only
the prelude (which disables interrupts & enables the flash mapping) is
in IRAM now.

Closes #54, relevant to #133.

core/newlib_syscalls.c

@@ -25,7 +25,7 @@ IRAM caddr_t _sbrk_r (struct _reent *r, int incr)
        if (heap_end + incr > stack_ptr)
        {
        _write (1, "_sbrk: Heap collided with stack\n", 32);
-       while(1) {}
+       abort();
        }
     */
     heap_end += incr;