Don't enable Hexagon by default unless LLVM_VERISON >= 3.9.
Don't enable Hexagon by default unless LLVM_VERISON >= 3.9.

psuriana/Halide,ronen/Halide,ronen/Halide,jiawen/Halide,psuriana/Halide,ronen/Halide,kgnk/Halide,jiawen/Halide,ronen/Halide,kgnk/Halide,ronen/Halide,ronen/Halide,kgnk/Halide,psuriana/Halide,jiawen/Halide,kgnk/Halide,kgnk/Halide,psuriana/Halide,kgnk/Halide,psuriana/Halide,ronen/Halide,jiawen/Halide,jiawen/Halide,jiawen/Halide,jiawen/Halide,ronen/Halide,psuriana/Halide,kgnk/Halide,kgnk/Halide,psuriana/Halide