Fix compilation warnings in the s390x architeture. Fixes #1082.

stb_sprintf.h

@@ -224,7 +224,7 @@ STBSP__PUBLICDEF void STB_SPRINTF_DECORATE(set_separators)(char comma, char peri
 #define stbsp__uint16 unsigned short
 
 #ifndef stbsp__uintptr
-#if defined(__ppc64__) || defined(__powerpc64__) || defined(__aarch64__) || defined(_M_X64) || defined(__x86_64__) || defined(__x86_64)
+#if defined(__ppc64__) || defined(__powerpc64__) || defined(__aarch64__) || defined(_M_X64) || defined(__x86_64__) || defined(__x86_64) || defined(__s390x__)
 #define stbsp__uintptr stbsp__uint64
 #else
 #define stbsp__uintptr stbsp__uint32