Merge _stub.o files into the main .o file (Fixes #3687 and #706)
Now GHC still generates the _stub.c files, but the object file is automatically merged into the main .o file for a module. This means that build systems (including GHC's own) no longer need to worry about looking for _stub.o files and including them when linking. I had to do lots of refactoring in DriverPipeline to make this work; now there's a monad to carry around all the information, and everything is a lot tidier. The _stub.c is now created as a temporary file and removed after compilation (unless the -keep-tmp-files flag is on).
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