We are about to allow passing Int128 to/from tcg helper functions,
but libffi doesn't support __int128_t, so use the structure.

In order for atomic128.h to continue working, we must provide
a mechanism to frob between real __int128_t and the structure.
Provide a new union, Int128Alias, for this.  We cannot modify
Int128 itself, as any changed alignment would also break libffi.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>