[PATCH][gold] Mips: Remove gold_unreachable from get_reference_flags

[Cary Coutant]

> gold_unreachable in Scan::get_reference_flags prevents gold to produce error if
> relocation is not supported.
>
>         * mips.cc (Target_mips::Scan::get_reference_flags): Remove
>         gold_unreachable from default case.

Committed on your behalf.

Thanks!

-cary