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/m_Do/m_Do_Reset.cpp

@@ -99,7 +99,7 @@ void mDoRst_reset(int reset, u32 resetCode, int forceMenu) {
 }
 
 void checkDiskCallback(s32 result, DVDCommandBlock* block) {
-    block->userData = (void*)result;
+    block->userData = (void*)(intptr_t)result;
 }
 
 void mDoRst_resetCallBack(int port, void*) {