tools: Add clean targets for tools directory

Adding missing clean targets for following tools directories:

  lib/bpf
  lib/subcmd
  build

This are now cleaned via 'make -C tools clean' command.

Reported-and-Tested-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1452509693-13452-2-git-send-email-jolsa@kernel.org


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