Add comment to amd64_pmc.c regarding Linux kernel's use of one of the performance monitor counters for CPU lockup detection.
Add comment to amd64_pmc.c regarding Linux kernel's use of one of the performance monitor counters for CPU lockup detection.

ubccr/tacc_stats,ubccr/tacc_stats,ubccr/tacc_stats,ubccr/tacc_stats,ubccr/tacc_stats