Implement the "mips endian" for r_info.

Implement the "mips endian" for r_info.

Normally r_info is just a 32 of 64 bit number matching the endian of the rest
of the file. Unfortunately, mips 64 bit little endian is special: The top 32
bits are a little endian number and the following 32 are a big endian one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178694 91177308-0d34-0410-b5e6-96231b3b80d8