[clang][bytecode] Only use int128 if it is available

This broke a number of buildbots, e.g.
https://lab.llvm.org/buildbot/#/builders/64/builds/4410

Author: tbaederr

clang/test/AST/ByteCode/intap.cpp

@@ -48,11 +48,12 @@ static_assert(DivA / DivB == 2, "");
 constexpr _BitInt(4) DivC = DivA / 0; // both-error {{must be initialized by a constant expression}} \
                                       // both-note {{division by zero}}
 
+#ifdef __SIZEOF_INT128__
 constexpr __int128 isMinDiv() {
   return __int128{0} / __int128{-1};
 }
 static_assert(isMinDiv() == 0, "");
-
+#endif
 
 
 constexpr _BitInt(7) RemA = 47;