selftests/openat2: Fix build warnings on ppc64

Fix warnings like:

  openat2_test.c: In function ‘test_openat2_flags’:
  openat2_test.c:303:73: warning: format ‘%llX’ expects argument of type
  ‘long long unsigned int’, but argument 5 has type ‘__u64’ {aka ‘long
  unsigned int’} [-Wformat=]

By switching to unsigned long long for u64 for ppc64 builds.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>