The old logic would lead to an error when
multiprocessing was enabled and
`LEON3_GPTIMER_BASE` was defined due to
`leon3_timer_core_index` being undefined.
The new logic fixes this and keeps the same
intent:
- If multiprocessing is not enabled, the timer
index is 0
- If multiprocessing is enabled and
`LEON3_GPTIMER_BASE` is defined, the timer
index is twice the CPU boot index
- If multiprocessing is enabled and
`LEON3_GPTIMER_BASE` is not defined, we
fallback to the old logic using the GPTIMER
core index.
Close#5258
