[mips] support FK_Data_2 and FK_Data_8 to fix big-endian debug data

This fixes invalid lengths in .debug_aranges on big-endian mips64
(lengths appear to be left-shifted by 32 bits) and in .debug_loc.

Differential Revision: http://llvm-reviews.chandlerc.com/D2517
[mips] Prevent %lo relocation being used on MSA loads and stores.

Summary:
Parts of the compiler still believed MSA load/stores have a 16-bit offset when
it is actually 10-bit. Corrected this, and fixed a closely related issue this
uncovered where load/stores with 10-bit and 12-bit offsets (MSA and microMIPS
respectively) could not load/store using offsets from the stack/frame pointer.
They accepted frameindex+offset, but not frameindex by itself.

Reviewers: jacksprat, matheusalmeida

Reviewed By: jacksprat

Differential Revision: http://llvm-reviews.chandlerc.com/D2888