perf trace: Prepare the strarray scnprintf method for reuse

Right now when an index passed to that method has no string associated
it'll print the index as a decimal number, prepare it so that we can use
it to print it in hex as well, for ioctls, for instance.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.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-nsvy06sqj64qvnkmzvwxsx2v@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>