Most-constrained-first greedy assigner.  pmu_count_core_hw_slots
enumerates global rows that are class-compatible with the leader and
pass amd_can_assign_pmc; pmu_assign_group sorts events by weight and
binds them all-or-none.  Critical detail: the assigner operates in
per-class adjri space for pe_cons.pc_allowed_rows / *used_mask /
pc_fixed_row, but the framework numbers PMC rows globally.  The code
converts adjri <-> ri via hwpmc_ri_to_classdep() and pcd->pcd_ri so
that amd_can_assign_pmc and pcd_allocate_pmc are always called with
the class-relative index they expect.  Without this conversion AMD
trips its own KASSERT (illegal row index >= amd_npmcs) on Zen, where
K8 rows start at global ri=17.

Add amd_can_assign_pmc() (factored out of amd_allocate_pmc) and
amd_get_sched_constraint() that emits a pmc_sched_constraint_t
covering every Zen sub-class (CORE / L3_CACHE / DATA_FABRIC).
pc_allowed_rows is built in the per-class adjri namespace as
documented in hwpmc_pmu.h.  The two new functions are prototyped in
hwpmc_pmu.h (alongside the rest of the PMU layer interface) so
hwpmc_amd.h needs no change.  Other backends (Intel/ARM) continue to
return EOPNOTSUPP so legacy non-grouped allocation is unaffected.