Implement superpages for PowerPC64 (HPT)

This change adds support for transparent superpages for PowerPC64
systems using Hashed Page Tables (HPT). All pmap operations are
supported.

The changes were inspired by RISC-V implementation of superpages,
by @markj (r344106), but heavily adapted to fit PPC64 HPT architecture
and existing MMU OEA64 code.

While these changes are not better tested, superpages support is disabled by
default. To enable it, use vm.pmap.superpages_enabled=1.

In this initial implementation, when superpages are disabled, system
performance stays at the same level as without these changes. When
superpages are enabled, buildworld time increases a bit (~2%). However,
for workloads that put a heavy pressure on the TLB the performance boost
is much bigger (see HPC Challenge and pgbench on D25237).

Reviewed by: jhibbits
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D25237