Avoid using godot_get_stack_bottom

It's not reliable on Android.

godot/godotinternal.nim

@@ -239,9 +239,6 @@ type ClassConstructor = proc (): ptr GodotObject {.noconv.}
 proc getClassConstructor*(className: cstring): ClassConstructor {.
   importc: "godot_get_class_constructor".}
 
-proc godotStackBottom*(): pointer {.
-  importc: "godot_get_stack_bottom".}
-
 proc deinit*(o: ptr GodotObject) {.
     importc: "godot_object_destroy".}
 

godot/nim/godotnim.nim

@@ -780,10 +780,10 @@ proc godot_nativescript_init(handle: pointer) {.
     cdecl, exportc, dynlib.} =
   nativeLibHandle = handle
 
-  let stackBottom = godotStackBottom()
+  var stackBottom {.volatile.}: pointer
   {.emit: """
     NimMain();
-    setStackBottom(`stackBottom`);
+    setStackBottom((void*)(&`stackBottom`));
   """.}
   GC_fullCollect()
   GC_disable()
@@ -798,6 +798,10 @@ proc godot_gdnative_terminate(options: ptr GodotNativeTerminateOptions) {.
 
 const nimGcStepLengthUs {.intdefine.} = 2000
 proc godot_nativescript_frame() {.cdecl, exportc, dynlib.} =
+  var stackBottom {.volatile.}: pointer
+  {.emit: """
+  setStackBottom((void*)(&`stackBottom`));
+  """.}
   if asyncdispatch.hasPendingOperations():
     poll(0)
   GC_step(nimGcStepLengthUs, true, 0)