HWPMC_HOOKS is defined. The pmc_cpu_is_*() functions in this file
are referenced unconditionally by hwpmc(4).
This is mostly a stop-gap. The pmc_cpu_is*() function should
probably be declared inline in <sys/pmc.h> or <sys/pmckern.h> and
the function pointers with corresponding SX lock should probably
be moved to another file and compiled conditionally upon HWPMC_HOOKS.
Ok'd by: jkoshy@