configure: Add signed*signed check to [u]int128_t test

clang 3.3 with -fsanitize=undefined will fail to link code containing an
int128_t * int128_t multiply (http://llvm.org/bugs/show_bug.cgi?id=16404)
so add this to our configure test for whether [u]int128_t are usable.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

configure

@@ -3329,6 +3329,7 @@ __uint128_t b;
 int main (void) {
   a = a + b;
   b = a * b;
+  a = a * a;
   return 0;
 }
 EOF