Some targets, notably AArch64 for ILP32, have different relocation encodings based upon the ABI. This is an enabling change, so a future patch can use the ABIName from MCTargetOptions to chose which relocations to use. Tested using check-llvm.
The corresponding change to clang is in: http://reviews.llvm.org/D16538
Does it matter that we don't initialize the ABI name here? I don't think it does but I thought I ought to double check.