Change detection logic for Objects-normal directory

Right now build_dir_output looks for a hard-coded directory named 'UnitTests.build'. Instead, use xcodebuild with some arguments to figure out where the files really ended up. I'm not sure if OBJECT_FILE_DIR_normal is a newer variable nor not, so I also had it fall back to TARGET_TEMP_DIR

Also, I'm not sure if you guys ran into it or not, but there are a lot of other tools in libphutil that make executing things a lot cleaner than just straight up using 'exec' (lots of injection prevention, cleaner results on error, etc). I didn't introduce any of that here, but just a heads up.