s390x: use CONFIG_INT128 to detect __uint128_t

Target s390x uses ad-hoc macro magic to guess if the compiler
supports the GCC extension __uint128_t.  This patch uses the
the dedicated macro CONFIG_INT128 defined by configure instead.

This fixes compilation with the CIL source code analyzer, which
uses GCC as a preprocessor but does not support __uint128_t.
Signed-off-by: NGabriel Kerneis <gabriel@kerneis.info>
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

target-s390x/int_helper.c

@@ -97,8 +97,7 @@ uint64_t HELPER(divu64)(CPUS390XState *env, uint64_t ah, uint64_t al,
         ret = al / b;
     } else {
         /* ??? Move i386 idivq helper to host-utils.  */
-#if HOST_LONG_BITS == 64 && defined(__GNUC__)
-        /* assuming 64-bit hosts have __uint128_t */
+#ifdef CONFIG_INT128
         __uint128_t a = ((__uint128_t)ah << 64) | al;
         __uint128_t q = a / b;
         env->retxl = a % b;