perf buildid-cache: Support --list option

'perf buildid-cache' allows to add/remove files into cache but there is
no option to list all cached files. Add --list option to list all
_valid_ cached files.

Ex,
  # perf buildid-cache --add /tmp/a.out
  # perf buildid-cache -l
    8a86ef73e44067bca52cc3f6cd3e5446c783391c /tmp/a.out

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Krister Johansen <kjlx@templeofstupid.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Sihyeon Jang <uneedsihyeon@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180417041346.5617-3-ravi.bangoria@linux.vnet.ibm.com


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