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_obj_flag.cpp

@@ -80,7 +80,7 @@ void daObjFlag_c::calcJointAngle() {
             joint->mJoint3 = joint->mJoint1 - joint->mJoint2;
         }
 
-        if(attr().field_0x25 != NULL) {
+        if(attr().field_0x25 != 0) {
             if(i == 1) {
                 calcAngleSwingX(joint, power);
             }