libpmc/hwpmc: fix issues with arm64 pmu-events support

Due to a mis-merge, the changes committed to libpmc never called
pmu_parse_event(), or set pm->pm_ev. However, this field shouldn't be
used to carry the actual pmc event code anyway, as it is expected to
contain the index into the pmu event array (otherwise, it breaks event
name lookup in pmclog_get_event()). Add a new MD field,
pm_md.pm_md_config, to pass the raw event code to arm64_allocate_pmc().

Additionally, the change made to pmc_md_op_pmcallocate was incorrect, as
this is a union, not a struct. Restore the proper padding size.

Reviewed by: luporl, ray, andrew
Fixes: 28dd6730a5d6 ("libpmc: enable pmu_utils on arm64")
Fixes: 8cc3815f02be ("hwpmc_arm64: accept raw event codes...")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31221