Set LG_VADDR to 48 on RISC-V.

The Sv48 PTE format is the largest currently defined address space for
RISC-V. It makes no sense to define a larger size and doing so (at
least for 64-bits) forces rtrees down a slow path.

Reviewed by: vangyzen, jhb, mhorne
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24658