Implement automatic parallel builds when building in tools/perf:

  $ time make

  # [ perf build: Doing 'make -j12' parallel build. ]

  Auto-detecting system features:

  ...

  real    0m9.265s
  user    0m59.888s
  sys     0m6.082s

On GNU make achieving this is not particularly easy, it requires a separate
makefile, which then invokes the main Makefile.

( Note: this patch adds Makefile.parallel to show the concept - the two
  makefiles will be flipped in the next patch to avoid having to specify -f
  to get parallelism in the default build. )

Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/n/tip-dvBjwqiTyzrufzkz8oanhpf9@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>