riscv: Maintain the allpmaps list only in SV39 mode

[markj]

Summary

When four-level page tables are used, there is no need to distribute
updates to the top-level page to all pmaps.

[jhb]

sys/riscv/riscv/pmap.c

1293

Maybe just leave this outside of the if-else?

[markj]

sys/riscv/riscv/pmap.c

1293

It looks slightly odd since I haven't rebased on D34267. Once that's done the redundancy will be gone.

[markj]

Rebase

[markj]

Copy only a single L0 entry in pmap_pinit().

[markj]

Rebase.