Add support for atomvm_free_heap_size key to system_info/1 on STM32 platform

Adds support for getting the current free heap size on STM32 platform using
`erlang:system_info(atomvm_free_heap_size)`.

Signed-off-by: Winford <winford@object.stream>

Author: UncleGrumpy

src/platforms/stm32/src/lib/sys.c

@@ -82,6 +82,11 @@ void *_sbrk_r(struct _reent *reent, ptrdiff_t diff)
     return _old_brk;
 }
 
+static int sys_get_free_heap()
+{
+    return (int) ((uint8_t *) (((uintptr_t) &_stack - RESERVE_STACK_SIZE))) - (_cur_brk == NULL ? (int) &_ebss : (int) _cur_brk);
+}
+
 // Monotonically increasing number of milliseconds from reset
 static volatile uint64_t system_millis;
 
@@ -263,11 +268,14 @@ Context *sys_create_port(GlobalContext *glb, const char *driver_name, term opts)
 
 term sys_get_info(Context *ctx, term key)
 {
-    UNUSED(ctx);
-    UNUSED(key);
+    GlobalContext *glb = ctx->global;
+    if (key == globalcontext_existing_term_from_atom_string(glb, ATOM_STR("\x15", "atomvm_free_heap_size"))) {
+        return term_from_int32(sys_get_free_heap());
+    }
     return UNDEFINED_ATOM;
 }
 
+
 void sys_enable_flash_cache()
 {
     flash_unlock_option_bytes();