Add a way to retrieve stack bottom of the main thread

I'm working on Nim bindings and Nim GC needs to know the stack
boundaries to check whether certain pointers are located on the stack
or in the heap. This commit adds godot_get_stack_bottom procedure
to gdnative module which returns pointer to the stack bottom of the
main thread. Later on this may be improved to return stack bottom of
the current thread.

modules/gdnative/godot/gdnative.cpp

@@ -33,6 +33,7 @@
 #include "error_macros.h"
 #include "gdnative.h"
 #include "global_constants.h"
+#include "os/os.h"
 #include "project_settings.h"
 #include "variant.h"
 
@@ -89,6 +90,10 @@ godot_object GDAPI *godot_global_get_singleton(char *p_name) {
 	return (godot_object *)ProjectSettings::get_singleton()->get_singleton_object(String(p_name));
 } // result shouldn't be freed
 
+void GDAPI *godot_get_stack_bottom() {
+	return OS::get_singleton()->get_stack_bottom();
+}
+
 // MethodBind API
 
 godot_method_bind GDAPI *godot_method_bind_get_method(const char *p_classname, const char *p_methodname) {

modules/gdnative/godot/gdnative.h

@@ -245,6 +245,10 @@ void GDAPI godot_object_destroy(godot_object *p_o);
 
 godot_object GDAPI *godot_global_get_singleton(char *p_name); // result shouldn't be freed
 
+////// OS API
+
+void GDAPI *godot_get_stack_bottom(); //  returns stack bottom of the main thread
+
 ////// MethodBind API
 
 typedef struct {