initial riscv SV48 mode support

Hi,

I added support for four-level page tables ("SV48 mode") to the riscv
pmap: https://reviews.freebsd.org/D34270 (see the "stack" tab for the
rest of the patches in the series).  So far only the user map layout is
changed.

SV48 mode may be useful for applications that want a larger virtual
address space than that provided by the SV39 ABI (where it's 256GB).
Currently SV48 is implemented by QEMU but not by extant hardware, and
the patches do not enable it by default.

Any comments on or reviews of the patches would be much appreciated.

Thanks,
-Mark