perf header: Store clock references for -k/--clockid option

Add a new CLOCK_DATA feature that stores reference times when
-k/--clockid option is specified.

It contains the clock id and its reference time together with wall clock
time taken at the 'same time', both values are in nanoseconds.

The format of data is as below:

  struct {
       u32 version;  /* version = 1 */
       u32 clockid;
       u64 wall_clock_ns;
       u64 clockid_time_ns;
  };

This clock reference times will be used in following changes to display
wall clock for perf events.

It's available only for recording with clockid specified, because it's
the only case where we can get reference time to wallclock time. It's
can't do that with perf clock yet.

Committer testing:

  $ perf record -h -k

   Usage: perf record [<options>] [<command>]
      or: perf record [<options>] -- <command> [<options>]

      -k, --clockid <clockid>
                            clockid to use for events, see clock_gettime()

  $ perf record -k monotonic sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.017 MB perf.data (8 samples) ]
  $ perf report --header-only | grep clockid -A1
  # event : name = cycles:u, , id = { 88815, 88816, 88817, 88818, 88819, 88820, 88821, 88822 }, size = 120, { sample_period, sample_freq } = 4000, sample_type = IP|TID|TIME|PERIOD, read_format = ID, disabled = 1, inherit = 1, exclude_kernel = 1, mmap = 1, comm = 1, freq = 1, enable_on_exec = 1, task = 1, precise_ip = 3, sample_id_all = 1, exclude_guest = 1, mmap2 = 1, comm_exec = 1, use_clockid = 1, ksymbol = 1, bpf_event = 1, clockid = 1
  # CPU_TOPOLOGY info available, use -I to display
  --
  # clockid frequency: 1000 MHz
  # cpu pmu capabilities: branches=32, max_precise=3, pmu_name=skylake
  # clockid: monotonic (1)
  # reference time: 2020-08-06 09:40:21.619290 = 1596717621.619290 (TOD) = 21931.077673635 (monotonic)
  $

Original-patch-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Geneviève Bastien <gbastien@versatic.net>
Cc: Ian Rogers <irogers@google.com>
Cc: Jeremie Galarneau <jgalar@efficios.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lore.kernel.org/lkml/20200805093444.314999-4-jolsa@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>