libpmc: make libpmc_pmu_utils.c more amenable to porting

The current version has every function stubbed out for !x86. Only two
functions (pmu_alias_get() and pmc_pmu_pmcallocate() are really platform
dependent, so reduce the width of the ifdefs and remove some of the
stubs.

Reviewed by: ray
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30532