Implement vm.pmap.kernel_maps for RISC-V

This is taken from the arm64 version, with the following simplifications:

• Our current pmap implementation uses a 3-level paging scheme
• The "mode" field has been omitted since RISC-V PTEs don't encode typical mode attributes

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D23594