plugins/cpu: Use simpler names when CPU core info is missing
Previously this would report "CPU 0 Core x" when cpu/core info is missing. Change it to just "Core x", just like the one-CPU case.
There's room here to better handle this for ARM platforms (e.g. displaying clusters and E/P core info), but for now this will do.
Signed-off-by: Hector Martin marcan@marcan.st