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

include/JSystem/JKernel/JKRHeap.h

@@ -5,6 +5,7 @@
 #include <dolphin/os.h>
 #include "global.h"
 #include <new>
+#include <cstdint>
 
 class JKRHeap;
 typedef void (*JKRErrorHandler)(void*, u32, int);
@@ -66,7 +67,7 @@ public:
     JKRHeap* find(void* ptr) const;
     JKRHeap* findAllHeap(void* ptr) const;
 
-    void dispose_subroutine(u32 start, u32 end);
+    void dispose_subroutine(uintptr_t start, uintptr_t end);
     bool dispose(void* ptr, u32 size);
     void dispose(void* begin, void* end);
     void dispose();