Calls `Streamer.setLFIRewriter` during generic LFIMCStreamer
initialization rather than requiring it to be done during
backend-specific initialization. This better follows the existing
conventions in `create*` functions in `TargetRegistry.h`.
Also re-adds the call to initSections for LFI in `llvm-mc.cpp`
(necessary in order to emit the ABI Note section), along with a test to
make sure ABI note emission with the rewriter is working.
e6e388cff0