perf buildid-list: Add option to show the running kernel build id

[root@emilia ~]# perf buildid-list -k
07b0c016a2b30004e86132d0239945b1e88f5d75

Useful when diagnosing build id problems in debuginfo packages, etc.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
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-po1bl7acn6e1hhne90opmvtl@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>