RISC-V superpage support, part 5/6.

[markj]

Summary

Add superpage support for pmap_ts_referenced(). While here, get rid of
!safe_to_clear_referenced() handling.

Per the privileged architecture spec, RISC-V implementations may set the
PTE_A bit in hardware (upon a TLB miss), or generate a page fault if the
implementation finds that PTE_A is not set. The latter scenario is
now handled by pmap_fault_fixup(), so we can simply clear the PTE_A bit.

[jhb]

sys/riscv/riscv/pmap.c

4052

Hmm, some other places used '2mpage' in comments, etc. It would be nice to be consistent overall. (I think I prefer the format here of '2MB page' and '4KB page'.)

[markj]

sys/riscv/riscv/pmap.c

4052

It's pretty inconsistent in general. This comment matches amd64 and arm64 at least.

[markj]

Committed in r344106.