Add definition for cpu_halt()

[mhorne063_gmail.com]

Summary

Add the missing definition cpu_halt() on RISC-V.

Test Plan

Tested using the halt command from ddb.

[markj]

sys/riscv/riscv/machdep.c

430 ↗

(On Diff #52138)

Per style(9) this should be:

for (;;)
        __asm __volatile("wfi");