riscv: Svpbmt extension support

The Svpbmt extension provides specification of "Page-Based Memory
Types", or memory attributes (e.g. cacheability constraints).

Extend the pmap code to apply memory attributes when creating/updating
PTEs. This is done in a way which has no effect on CPUs lacking Svpbmt
support, and is non-hostile to alternate encodings of memory attributes

• a future change will enable this for T-HEAD CPUs, which implement

this PTE feature in an different (incompatible) way.

Reviewed by: jhb
Tested by: br
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45471