selftests: cachestat: Fix build warnings on ppc64

Fix warnings like:
  test_cachestat.c: In function ‘print_cachestat’:
  test_cachestat.c:30:38: warning: format ‘%llu’ expects argument of
  type ‘long long unsigned int’, but argument 2 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>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>