stm32: In link script, define start of stack separately from heap end.

Previously the end of the heap was the start (lowest address) of the stack.
With the changes in this commit these addresses are now independent,
allowing a board to place the heap and stack in separate locations.

ports/stm32/main.c

@@ -536,7 +536,7 @@ soft_reset:
     // to recover from limit hit.  (Limit is measured in bytes.)
     // Note: stack control relies on main thread being initialised above
     mp_stack_set_top(&_estack);
-    mp_stack_set_limit((char*)&_estack - (char*)&_heap_end - 1024);
+    mp_stack_set_limit((char*)&_estack - (char*)&_sstack - 1024);
 
     // GC init
     gc_init(MICROPY_HEAP_START, MICROPY_HEAP_END);