mips/malta: Prefer _start over _locore for entry point symbol

These are aliases, but it's probably nicer to use the same symbol as we
use for the actual ELF entry point.

This is a direct commit to stable/13 as mips no longer exists in main.

(cherry picked from commit e32b14ef10a7b0c3130ead9e676c8e6f261f0b69)