Implement transparent 2MB superpage promotion for RISC-V.

This includes support for pmap_enter(..., psind=1) as described in the
commit log message for r321378.

The changes are largely modelled after amd64. arm64 has more stringent
requirements around superpage creation to avoid the possibility of TLB
conflict aborts, and these requirements do not apply to RISC-V, which
like amd64 permits simultaneous caching of 4KB and 2MB translations for
a given page. RISC-V's PTE format includes only two software bits, and
as these are already consumed we do not have an analogue for amd64's
PG_PROMOTED. Instead, pmap_remove_l2() always invalidates the entire
2MB address range.

pmap_ts_referenced() is modified to clear PTE_A, now that we support
both hardware- and software-managed reference and dirty bits. Also
fix pmap_fault_fixup() so that it does not set PTE_A or PTE_D on kernel
mappings.

Reviewed by: kib (earlier version)
Discussed with: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18863
Differential Revision: https://reviews.freebsd.org/D18864
Differential Revision: https://reviews.freebsd.org/D18865
Differential Revision: https://reviews.freebsd.org/D18866
Differential Revision: https://reviews.freebsd.org/D18867
Differential Revision: https://reviews.freebsd.org/D18868