MFC r239591:

Remove unncessary atomic operation when reading process flags in
PMC_PROC_IS_USING_PMCS macro.

Invocations of this macro are not synchronized with setting/clearing
of P_HWPMC flag, so the atomic operation here isn't needed. Removing
the atomic operation provides noticeable improvement (5-6%) on
some scheduler-intensive workloads with HWPMC_HOOKS enabled on an
8C Sandy Bridge Xeon system.

Sponsored by: Intel