tools/perf/build: Split out feature check: 'backtrace'

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-ihnwe6cvglVkudyvcavP1wql@git.kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>