riscv: S-mode extension parsing

There are now several Supervisor-mode extensions that have entered the
'ratified' status, so begin parsing and reporting a few of these.

Recognize the following extensions:

• Sstc: stimecmp/vstimecmp CSR
• Svnapot: NAPOT* translation contiguity
• Svpbmt: page-based memory types
• Svinval: fine-grained TLB invalidation instructions
• Sscofpmf: performance counter overflow

*i.e. "naturally aligned power-of-2" page granularity

For now, provide globals for Sstc and Sscofpmf, as we will make use of
these in the near future.

Plus, update the copyright statement after my recent work on this file.

Reviewed by: jhb
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40240