perf tools: Merge deferred user callchains

perf tools: Merge deferred user callchains

Save samples with deferred callchains in a separate list and deliver
them after merging the user callchains.  If users don't want to merge
they can set tool->merge_deferred_callchains to false to prevent the
behavior.

With previous result, now perf script will show the merged callchains.

  $ perf script
  ...
  pwd    2312   121.163435:     249113 cpu/cycles/P:
          ffffffff845b78d8 __build_id_parse.isra.0+0x218 ([kernel.kallsyms])
          ffffffff83bb5bf6 perf_event_mmap+0x2e6 ([kernel.kallsyms])
          ffffffff83c31959 mprotect_fixup+0x1e9 ([kernel.kallsyms])
          ffffffff83c31dc5 do_mprotect_pkey+0x2b5 ([kernel.kallsyms])
          ffffffff83c3206f __x64_sys_mprotect+0x1f ([kernel.kallsyms])
          ffffffff845e6692 do_syscall_64+0x62 ([kernel.kallsyms])
          ffffffff8360012f entry_SYSCALL_64_after_hwframe+0x76 ([kernel.kallsyms])
              7f18fe337fa7 mprotect+0x7 (/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2)
              7f18fe330e0f _dl_sysdep_start+0x7f (/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2)
              7f18fe331448 _dl_start_user+0x0 (/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2)
  ...

The old output can be get using --no-merge-callchain option.
Also perf report can get the user callchain entry at the end.

  $ perf report --no-children --stdio -q -S __build_id_parse.isra.0
  # symbol: __build_id_parse.isra.0
       8.40%  pwd      [kernel.kallsyms]
              |
              ---__build_id_parse.isra.0
                 perf_event_mmap
                 mprotect_fixup
                 do_mprotect_pkey
                 __x64_sys_mprotect
                 do_syscall_64
                 entry_SYSCALL_64_after_hwframe
                 mprotect
                 _dl_sysdep_start
                 _dl_start_user

Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>