perf/core improvements and fixes:

Improvements:

- Improve error message when analyzing file with required events in
  'perf sched timehist' (David Ahern)

Fixes:

- Force fixdep compilation to be done at the start of the build, fixing
  some build race conditions in high core count machines (Jiri Olsa)

- Fix handling a zero sample->tid in 'perf sched timehist', as
  sometimes that isn't the idle thread (Namhyung Kim)

Infrastructure:

- Check minimal accepted LLVM version in its feature check, 3.9 at this
  time (Wang Nan)

Documentation:

- Explicitly document that --children is enabled by default (Yannick Brosseau)

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