make nqp::isbig_I consistent with set_int and get_int

turns out that set_int and get_int only support 32bit, even on 64bit
platforms. While the better fix would be to make them work on 64 bit width,
this approach change nqp::isbig_i to return 1 if the stored int takes more
than 32bit.

src/ops/nqp_bigint.ops

@@ -410,6 +410,11 @@ inline op nqp_bigint_pow(out PMC, in PMC, in PMC, in PMC) :base_core {
 /* returns 1 if $2 is too large to fit into an INTVAL without loss of
    information */
 inline op nqp_bigint_is_big(out INT, in PMC) :base_core {
-    mp_int  *a = get_bigint(interp, $2);
+    mp_int *a = get_bigint(interp, $2);
     $1 = a->used > 1;
+    if ($1 == 0) {
+        /* mp_{set,get}_int is limited to 32 bit, so be extra careful. */
+        if (DIGIT(a, 0) & (~0xFFFFFFFFUL))
+            $1 = 1;
+    }
 }