More GCC compatibility/warning fixes (#3118)

* Wrap >4-char literals in a MULTI_CHAR macro

Modern compilers do not support CW's non-standard behavior with
>4 char literals. We can, however, use a constexpr function to
compute the u64 values directly. This leaves <=4 char literals
unchanged.

* Replace non-pointer usages of NULL with 0

* Define NULL to nullptr on C++11 and above

* Fix more -Wpointer-arith and -Woverflow warnings

* Replace u32/s32 with uintptr_t/intptr_t where appropriate

* JSUOutputStream: Overload all standard int types

src/d/actor/d_a_e_bg.cpp

@@ -93,7 +93,7 @@ int daE_BG_c::ctrlJoint(J3DJoint* i_joint, J3DModel* i_model) {
 }
 
 int daE_BG_c::JointCallBack(J3DJoint* i_joint, int param_1) {
-    if (param_1 == NULL) {
+    if (param_1 == 0) {
         J3DModel* model = j3dSys.getModel();
         daE_BG_c* bg = (daE_BG_c*)model->getUserArea();
         if (bg != NULL) {