[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

llvm-svn: 202716