riscv: Implement atomic operations

Implement atomic_load_acq_16, atomic_store_rel_16.

These are needed by bhyve(8) PCIe bus emulation code.

Group 16-bit atomic functions similarly to 32 and 64-bit.

Reviewed by: mhorne
Differential Revision: https://reviews.freebsd.org/D45228

(cherry picked from commit 2183004e14a863d65ca9192df1bea2625e3fdfb3)