riscv vmm: add SSTC extension check.

Check if RISC-V SSTC is available and advertise to the guest.

This is needed for Eswin EIC7700 that does not include SSTC.

As we don't have a mechanism for reporting extension presence
from the kernel to userspace, then use vm_cap_type for now.

Reviewed by: mhorne, markj
Differential Revision: https://reviews.freebsd.org/D48058