llvm-lto: default Relocation Model should be selected by the TargetMachine.

Right now, the createTargetMachine function in LTOBackend.cpp (used by llvm-lto, and other components) selects the default Relocation Model when none is specified in the module.
Other components (such as opt and llc) that construct a TargetMachine delegate the decision on the default value to the polymorphic TargetMachine's constructor.

This commit aligns llvm-lto with other components.

Reviewed By: daltenty, fhahn

Differential Revision: https://reviews.llvm.org/D97507