perf tools: Add support for the xtensa architecture

Tested using kernel tracepoints on a QEMU simulated environment.

Kernel support for perf depends on the patch "xtensa: enable
HAVE_PERF_EVENTS", which is scheduled for v3.14.

Hardware performance counters are not supported under xtensa yet.

Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: linux-xtensa@linux-xtensa.org
Link: http://lkml.kernel.org/r/aafcdb22f04e2d3188d2938528939481be56b649.1389608855.git.baruch@tkos.co.il


Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>