MIPS: Remove cc-option checks for -march=octeon

Nowadays our minimal supported GCC/Clang all support -march=octeon.
Remove cc-option checks to simplify code.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>