hwpmc: fix performance issues

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;