tools/power turbostat: allow running without cpu0

Linux-3.7 added CONFIG_BOOTPARAM_HOTPLUG_CPU0,
allowing systems to offline cpu0.

But when cpu0 is offline, turbostat will not run:

 # turbostat ls
turbostat: no /dev/cpu/0/msr

This patch replaces the hard-coded use of cpu0 in turbostat
with the current cpu, allowing it to run without a cpu0.

Fewer cross-calls may also be needed due to use of current cpu,
though this hard-coding was used only for the --debug preamble.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>