Fix data relocations for big-endian MIPS

Add entries for FK_Data_* data fixups to fix debug data.

The code in MipsAsmBackend::applyFixup requires the full fixup size for
big-endian targets, but included a switch statement containing only the
fixup sizes for two MIPS-specific types.

A future commit will clean up this implementation further, and should
use llvm_unreachable() for the default case to prevent this issue from
recurring.