a quick fixed posted on Apple Dev forums to get Xcode 7 working on DP Beta 6 build:
cd /Applications/Xcode-beta.app (or wherever your Xcode 7 beta 4 is located)
cd Contents/Developer/Platforms/iPhoneSimulator.platform
cd Developer/SDKs/iPhoneSimulator.sdk/usr/lib
sudo mv dyld_sim dyld_sim.orig
tested it myself and it works.