arch/ppc: Fix get_cpu_clock asm clobber list

Mark condition register 0 (cr0) as being clobbered by the inline asm in
get_cpu_clock(). Not doing this results in strange behaviour due to GCC
optimising away some checking. For example e.g:

  $ ./fio examples/null.fio --output-format=json
  time     5500  cycles_start=8085227422910      <--- bad!
  {
    "fio version" : "fio-2.21-89-gb034",
    <snip>

The extra output is due to the flag checking in dprint being removed
at higher optimisation levels.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>