tools/cpupower: Choose base_cpu to display default cpupower details

The default output of cpupower info utils shows unexpected output
when CPU 0 is disabled.

Considering a case where CPU 0 is disabled, output of cpupower idle-info:

Before change:
cpupower idle-info
CPUidle driver: pseries_idle
CPUidle governor: menu
analyzing CPU 0:
 *is offline

After change:
./cpupower idle-info
CPUidle driver: pseries_idle
CPUidle governor: menu
analyzing CPU 50:

Number of idle states: 2
Available idle states: snooze CEDE
snooze:
Flags/Description: snooze
Latency: 0
Usage: 101748
Duration: 2724058
CEDE:
Flags/Description: CEDE
Latency: 12
Usage: 270004
Duration: 283019526849

If -c option is not passed, CPU 0 was chosen as the default chosen CPU to
display details. However when CPU 0 is offline, it results in showing
unexpected output. This commit chooses the base_cpu
instead of CPU 0, hence keeping the output more relevant in all cases.
The base_cpu is the number of CPU on which the calling thread is
currently executing.

Signed-off-by: Saket Kumar Bhaskar <skb99@linux.vnet.ibm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>