Fix a bug in resolving HI16/LO16 relocation pairs for MIPS

[Cary Coutant]

> Attached is the patch that fixes a bug in resolving HI16/LO16 relocation pairs for MIPS. Instead of using symbol value to find HI16/LO16 pairs, the patch uses symbol index and relocation type.

Thanks! I've committed the patch on your behalf.

Sorry, I forgot to use the git commit --author option. You should ask
for write access, if you haven't already.

-cary