tools lib api cpu: Introduce cpu.[ch] to obtain cpu related information

E.g.:

 $ ./cpu__get_max_freq
 3200000

It does that, as Kan's patch does, by looking at these files:

  $ cat /sys/devices/system/cpu/online
  0-3
  $ ./sysfs__read_ull
  devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freq
  /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freq=3200000
  $

I.e. find out the first online CPU, then read its cpufreq info.

But do it in tools/lib/api/, so that other tools/ living code can use
it, not just perf.

Based-on-a-patch-by: Kan Liang <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Wang Nan <[email protected]>
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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