perf buildid-cache: Add option to show build ids that are missing in the cache

This will allow to connect with services being put in place by distros such as
Fedora, where one can retrieve DSOs by their build-id.

Example usage:

	for buildid in $(perf buildid-cache --missing perf.data | cut -d' ' -f1) ; do
		echo "trying to get $buildid"
		wget -q https://darkserver.fedoraproject.org/buildids/$buildid
		cat $buildid ; echo
		rm -f $buildid
	done

Now its just a matter of some porcelain to get the details provided by such a
service, retrieve the file and use 'perf buildid-cache --add $FILE' to insert
it in the cache, then use 'perf report' or 'annotate' that will find the
required files in the cache.

More information about the darkserver service at:

	https://darkserver.fedoraproject.org/

Cc: David Ahern <dsahern@gmail.com>
Cc: Frank Eigler <fche@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kushal Das <kdas@redhat.com>
Cc: Mark Wielaard <mjw@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-6fuktuiyjn4jykxmt7c9f7xq@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>