riscv: Make sure local hart's icache is synced in pmap_sync_icache

The only way to flush the local hart's icache is with a FENCE.I (or an
equivalent SBI call); a normal FENCE is insufficient and, for the
single-hart case, unnecessary.

Reviewed by: jhb (mentor), markj
Approved by: jhb (mentor), markj
Differential Revision: https://reviews.freebsd.org/D24317