perf/core improvements and fixes:

User visible:

- Order output of 'perf trace --summary' better, now the threads will
  appear ascending order of number of events, and then, for each, in
  descending order of syscalls by the time spent in the syscalls, so
  that the last page produced can be the one about the most interesting
  thread straced, suggested by Milian Wolff (Arnaldo Carvalho de Melo)

- Do not show the runtime_ms for a thread when not collecting it, that
  is done so far only with 'perf trace --sched' (Arnaldo Carvalho de Melo)

- Fix kallsyms perf test on ppc64le (Naveen N. Rao)

Infrastructure:

- Move global variables related to presence of some keys in the sort order to a
  per hist struct, to allow code like the hists browser to work with multiple
  hists with different lists of columns (Jiri Olsa)

- Add support for generating bpf prologue in powerpc (Naveen N. Rao)

- Fix kprobe and kretprobe handling with kallsyms on ppc64le (Naveen N. Rao)

- evlist mmap changes, prep work for supporting reading backwards (Wang Nan)

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