tcg: Use HAVE_CMPXCHG128 instead of CONFIG_CMPXCHG128

We adjust CONFIG_ATOMIC128 and CONFIG_CMPXCHG128 with
CONFIG_ATOMIC128_OPT in atomic128.h.  It is difficult
to tell when those changes have been applied with the
ifdef we must use with CONFIG_CMPXCHG128.  So instead
use HAVE_CMPXCHG128, which triggers -Werror-undef when
the proper header has not been included.

Improves tcg_gen_atomic_cmpxchg_i128 for s390x host, which
requires CONFIG_ATOMIC128_OPT.  Without this we fall back
to EXCP_ATOMIC to single-step 128-bit atomics, which is
slow enough to cause some tests to time out.

Reported-by: Thomas Huth <thuth@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>