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/JUtility/JUTXfb.h

@@ -75,7 +75,7 @@ private:
     static JUTXfb* sManager;
 
 private:
-    /* 0x00 */ void* mBuffer[3];
+    /* 0x00 */ u8* mBuffer[3];
     /* 0x0C */ bool mXfbAllocated[3];
     /* 0x10 */ s32 mBufferNum;
     /* 0x14 */ s16 mDrawingXfbIndex;