bindings/go: reinstate TargetMachine.TargetData

Summary:
LLVMGetTargetDataLayout was removed from the C API,
and then TargetMachine.TargetData was removed. Later,
LLVMCreateTargetMachineData was added to the C API,
and we now expose this via the Go API.

Reviewers: deadalnix, pcc

Subscribers: cierniak, llvm-commits, axw

Differential Revision: http://reviews.llvm.org/D18173

llvm-svn: 263530