Add a new report to display a call tree. The Call Tree report is very
similar to the Context-Sensitive Call Graph, but the data is not
aggregated. Also the 'Count' column, which would be always 1, is replaced
by the 'Call Time'.

Committer testing:

  $ cat simple-retpoline.c
  /*

    https://lkml.kernel.org/r/20190109091835.5570-6-adrian.hunter@intel.com

  $ gcc -ggdb3 -Wall -Wextra -O2 -o simple-retpoline simple-retpoline.c
  $ objdump -d simple-retpoline
  */

  __attribute__((noinline)) int bar(void)
  {
          return -1;
  }

  int foo(void)
  {
          return bar() + 1;
  }

  __attribute__((indirect_branch("thunk"))) int main()
  {
          int (*volatile fn)(void) = foo;

          fn();
          return fn();
  }
  $
  $ perf record -o simple-retpoline.perf.data -e intel_pt/cyc/u ./simple-retpoline
  $ perf script -i simple-retpoline.perf.data --itrace=be -s ~acme/libexec/perf-core/scripts/python/export-to-sqlite.py simple-retpoline.db branches calls
  $ python ~acme/libexec/perf-core/scripts/python/exported-sql-viewer.py simple-retpoline.db

And in the GUI select:

    "Reports"
      "Call Tree"

    Call Path                 | Object          | Call Time (ns) | Time (ns) | Time (%) | Branch Count | Brach Count (%) |
    > simple-retpolin
      > PID:TID
        > _start                ld-2.28.so       2193855505777      156267      100.0       10602          100.0
            unknown             unknown          2193855506010        2276        1.5           1            0.0
          > _dl_start           ld-2.28.so       2193855508286      137047       87.7       10088           95.2
          > _dl_init            ld-2.28.so       2193855645444        9142        5.9         326            3.1
          > _start              simple-retpoline 2193855654587        7457        4.8         182            1.7
            > __libc_start_main <SNIP>
              <SNIP>
              > main            simple-retpoline 2193855657493          32        0.5          12            6.7
                > foo           simple-retpoline 2193855657493          14       43.8           5           41.7
              <SNIP>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: https://lkml.kernel.org/n/tip-enf0w96gqzfpv4fi16pw9ovc@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>