meson: Fix detect atomic128 support with optimization

Silly typo: sizeof(16) != 16.

Fixes: e61f1ef ("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>

meson.build

@@ -2557,7 +2557,7 @@ if has_int128
   # __alignof(unsigned __int128) for the host.
   atomic_test_128 = '''
     int main(int ac, char **av) {
-      unsigned __int128 *p = __builtin_assume_aligned(av[ac - 1], sizeof(16));
+      unsigned __int128 *p = __builtin_assume_aligned(av[ac - 1], 16);
       p[1] = __atomic_load_n(&p[0], __ATOMIC_RELAXED);
       __atomic_store_n(&p[2], p[3], __ATOMIC_RELAXED);
       __atomic_compare_exchange_n(&p[4], &p[5], p[6], 0, __ATOMIC_RELAXED, __ATOMIC_RELAXED);