'feature_timerfd' is checked all the time and calculated explicitly,
in a serial fashion. Add it to CORE_FEATURE_TESTS which causes it to
be built in parallel, using the newfangled parallel build autodetection
code.

This shaves 137 msecs off the perf build time on my system, which
speeds up the common case cached build by 43%:

Before:

  comet:~/tip> perf stat --null --repeat 5 make -C tools/perf/
  [...]
         0,453771441 seconds time elapsed                                          ( +-  0,09% )

After:

  comet:~/tip> perf stat --null --repeat 5 make -C tools/perf/
  [...]
         0,316290185 seconds time elapsed                                          ( +-  0,24% )

Cc: David Ahern <dsahern@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/n/tip-bb92CmexihopoSyqnkqepvsy@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>