Differential revision:	https://reviews.freebsd.org/D32025

Avoid using atomics as it_wait is guarded by td_lock.

Report threshold calculation is done only if at least one PMC hook
is installed

Fixes:
* avoid unnecessary branching (if frame != null ...)
  by having PMC_HOOK_INSTALLED_ANY
  condition on the top of them, which should hint
  the core not to execute speculatively anything
  which us underneath;
* access intr_hwpmc_waiting_report_threshold cacheline
  only if at least one hook is loaded;