perf trace: Allow specifying index offset in strarrays

So that the index passed doesn't have to start at zero, being
decremented from an offset specified when declaring the strarray before
being used as the real array index.

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-k1ce6uqyt4qar9edrj3mevod@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>