[MIPS] Makefile: Fix canonical system names

The GNU `config.guess' uses "linux-gnu" as the canonical system name.
Fix the list of compiler prefixes checked to spell it correctly.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>