s390x: use CONFIG_INT128 to detect __uint128_t

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: Gabriel Kerneis <gabriel@kerneis.info>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>