powerpc/perf: Build callchain code regardless of hardware event support.

It's also useful for software events, as well as future support for
other types of hardware counters.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>