perf tools: Add cat as fallback pager

This patch adds a fallback to cat for the pager. This is useful
on environments, such as Android, where less does not exist.
It is better to default to cat than to abort.

Signed-off-by: Michael Lentine <mlentine@google.com>
Reviewed-by: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1400579330-5043-2-git-send-email-eranian@google.com
Signed-off-by: Jiri Olsa <jolsa@kernel.org>