Switch back to 3.7.1 for mac because there is no 3.8.0 llvm pre-release for mac.
Switch back to 3.7.1 for mac because there is no 3.8.0 llvm pre-release for mac.

trailofbits/remill,trailofbits/remill,trailofbits/remill,trailofbits/remill