MFC r312724,r312901,r312902:

hwpmc: partially depessimize munmap handling if the module is not loaded

HWPMC_HOOKS is enabled in GENERIC and triggers some work avoidable in the
common (module not loaded) case.

In particular this avoids permission checks + lock downgrade
singlethreaded and in cases were an executable mapping is found the pmc
sx lock is no longer bounced.

Note this is a band aid.

hwpmc: partially depessimize mmap handling if the module is not loaded

In particular this means the pmc sx lock is no longer taken when an
executable mapping succeeds.

hwpmc: annotate pmc_hook and pmc_intr as __read_mostly