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_nbomb.cpp

@@ -97,7 +97,7 @@ int daNbomb_c::searchEnemy(fopAc_ac_c* i_enemy) {
 
 static void* daNbomb_searchEnemy(fopAc_ac_c* i_actor, void* i_data) {
     if (fopAcM_GetGroup(i_actor) == fopAc_ENEMY_e &&
-        ((daNbomb_c*)i_data)->searchEnemy(i_actor) != NULL)
+        ((daNbomb_c*)i_data)->searchEnemy(i_actor) != 0)
     {
         return i_actor;
     }