riscv: Introduce SBI IPI convenience functions

The SBI IPI function interface is a bit painful to use since it
operates on hartids as opposed to cpuids and requires determining a
mask base and a mask. Provide functions allowing IPIs to be sent to
single cpus and to all cpus set in a cpumask in order to simplify
things for unit tests.

Signed-off-by: Andrew Jones <andrew.jones@linux.dev>