[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202716 91177308-0d34-0410-b5e6-96231b3b80d8

(This is a cherry-pick from upstream LLVM to address the loss of
FK_Data_2 in a previous rebase.)