[IA64] alternate perfmon handler

Patch from Charles Spirakis

Some linux customers want to optimize their applications on the latest
hardware but are not yet willing to upgrade to the latest kernel. This
patch provides a way to plug in an alternate, basic, and GPL'ed PMU
subsystem to help with their monitoring needs or for specialty work. It
can also be used in case of serious unexpected bugs in perfmon. Mutual
exclusion between the two subsystems is guaranteed, hence no conflict
can arise from both subsystem being present.

Acked-by: Stephane Eranian <eranian@hpl.hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>