Fix for bug #29079: Semantics of "bigint" depend on platform specifics (size, signedness of char ?)

Problem: long and long long types mess in a comparison may lead to wrong results on some platforms.
Fix: prefer [unsigned] long long as [u]longlong as it's used unconditionally in many places.

include/my_global.h

@@ -828,7 +828,12 @@ error "Neither int or long is of 4 bytes width"
 typedef unsigned long	ulong;		  /* Short for unsigned long */
 #endif
 #ifndef longlong_defined
-#if defined(HAVE_LONG_LONG) && SIZEOF_LONG != 8
+/* 
+  Using [unsigned] long long is preferable as [u]longlong because we use 
+  [unsigned] long long unconditionally in many places, 
+  for example in constants with [U]LL suffix.
+*/
+#if defined(HAVE_LONG_LONG) && SIZEOF_LONG_LONG == 8
 typedef unsigned long long int ulonglong; /* ulong or unsigned long long */
 typedef long long int	longlong;
 #else