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/c/c_dylink.cpp

@@ -857,7 +857,7 @@ BOOL cCc_Check() {
     BOOL rt = TRUE;
     for (int i = 0; i < ARRAY_SIZEU(DynamicNameTable); i++) {
         uintptr_t ptr = (uintptr_t)DMC[i];
-        if (ptr != NULL) {
+        if (ptr != 0) {
             u32 ptr_hi_byte = ptr >> 0x18;
             if (ptr_hi_byte < 0x80 || ptr_hi_byte > 0x83) {
                 // "cCc_Check invalid pointer detected"