riscv timer: use stimecmp CSR when available

The Sstc extension defines a new stimecmp CSR, allowing supervisor
software to set the timer, rather than just read it. When supported,
using this avoids the frequent trips through the SBI every time the
CPU's timer expires.

Reviewed by: jhb
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40241