perf header: Support CPU PMU capabilities

To stitch LBR call stack, the max LBR information is required. So the
CPU PMU capabilities information has to be stored in perf header.

Add a new feature HEADER_CPU_PMU_CAPS for CPU PMU capabilities.
Retrieve all CPU PMU capabilities, not just max LBR information.

Add variable max_branches to facilitate future usage.

Committer testing:

  # ls -la /sys/devices/cpu/caps/
  total 0
  drwxr-xr-x. 2 root root    0 Apr 17 10:53 .
  drwxr-xr-x. 6 root root    0 Apr 17 07:02 ..
  -r--r--r--. 1 root root 4096 Apr 17 10:53 max_precise
  #
  # cat /sys/devices/cpu/caps/max_precise
  0
  # perf record sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.033 MB perf.data (7 samples) ]
  #
  # perf report --header-only | egrep 'cpu(desc|.*capabilities)'
  # cpudesc : AMD Ryzen 5 3600X 6-Core Processor
  # cpu pmu capabilities: max_precise=0
  #

And then on an Intel machine:

  $ ls -la /sys/devices/cpu/caps/
  total 0
  drwxr-xr-x. 2 root root    0 Apr 17 10:51 .
  drwxr-xr-x. 6 root root    0 Apr 17 10:04 ..
  -r--r--r--. 1 root root 4096 Apr 17 11:37 branches
  -r--r--r--. 1 root root 4096 Apr 17 10:51 max_precise
  -r--r--r--. 1 root root 4096 Apr 17 11:37 pmu_name
  $ cat /sys/devices/cpu/caps/max_precise
  3
  $ cat /sys/devices/cpu/caps/branches
  32
  $ cat /sys/devices/cpu/caps/pmu_name
  skylake
  $ perf record sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.001 MB perf.data (8 samples) ]
  $ perf report --header-only | egrep 'cpu(desc|.*capabilities)'
  # cpudesc : Intel(R) Core(TM) i5-7500 CPU @ 3.40GHz
  # cpu pmu capabilities: branches=32, max_precise=3, pmu_name=skylake
  $

Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Pavel Gerasimov <pavel.gerasimov@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Vitaly Slobodskoy <vitaly.slobodskoy@intel.com>
Link: http://lore.kernel.org/lkml/20200319202517.23423-3-kan.liang@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>