meson: Split test for __int128_t type from __int128_t arithmetic

meson: Split test for __int128_t type from __int128_t arithmetic

Older versions of clang have missing runtime functions for arithmetic
with -fsanitize=undefined (see 464e3671f9d5c), so we cannot use
__int128_t for implementing Int128.  But __int128_t is present,
data movement works, and it can be used for atomic128.

Probe for both CONFIG_INT128_TYPE and CONFIG_INT128, adjust
qemu/int128.h to define Int128Alias if CONFIG_INT128_TYPE,
and adjust the meson probe for atomics to use has_int128_type.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>