riscv: translate CPU numbers to hartid values for IPI

Given the Zephyr CPU number is no longer tied to the hartid, we must
consider the actual hartid when sending an IPI to a given CPU. Since
those hartids can be anything, let's just save them in the cpu structure
as each CPU is brought online.

While at it, throw in some `get_hart_msip()` cleanups.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>