perf events, x86: Add Westmere stalled-cycles-frontend/backend events

Extend the Intel Westmere PMU driver with definitions for generic front-end and
back-end stall events.

( These are only approximations. )

Reported-by: David Ahern <dsahern@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/n/tip-7y40wib8n008io7hjpn1dsrm@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>