Silly typo: sizeof(16) != 16.

Fixes: e61f1efe ("meson: Detect atomic128 support with optimization")
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>