[LLVM-C] Expand LLVMRelocMode

Summary: Add read[only|write] PIC relocation models to the C API and teach the TargetMachine API about it.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56187

llvm-svn: 350279