perf script: Use fallbacks for branch stacks

perf script: Use fallbacks for branch stacks

Branch stacks do not necessarily have the same cpumode as the 'ip'. Use
the fallback functions in those cases.

This patch depends on patch "perf tools: Add fallback functions for cases
where cpumode is insufficient".

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: stable@vger.kernel.org # 4.19
Link: http://lkml.kernel.org/r/20181106210712.12098-4-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>