meson: fix CONFIG_ATOMIC128 check

meson: fix CONFIG_ATOMIC128 check

The current testing code isn't correct and matching QEMU usage:

testfile.c: In function 'main':
testfile.c:5:11: error: incorrect number of arguments to function '__atomic_load'
    5 |       y = __atomic_load(&x, 0);
      |           ^~~~~~~~~~~~~
testfile.c:6:7: error: argument 2 of '__atomic_store' must be a pointer type
    6 |       __atomic_store(&x, y, 0);
      |       ^~~~~~~~~~~~~~
testfile.c:7:7: error: argument 3 of '__atomic_compare_exchange' must be a pointer type
    7 |       __atomic_compare_exchange(&x, &y, x, 0, 0, 0);
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~

Replace the test with common atomics test for u64 and u128 that matches
better QEMU needs.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>