Building on ARMV8 platform I get:
[ 29%] Building CXX object modules/dnn/3rdparty/protobuf/CMakeFiles/libprotobuf.dir/src/google/protobuf/descriptor.cc.o
In file included from /home/test/opencv_contrib-3.1.0/modules/dnn/3rdparty/protobuf/src/google/protobuf/stubs/atomicops.h:59:0,
from /home/test/opencv_contrib-3.1.0/modules/dnn/3rdparty/protobuf/src/google/protobuf/stubs/once.h:81,
from /home/test/opencv_contrib-3.1.0/modules/dnn/3rdparty/protobuf/src/google/protobuf/descriptor.cc:53:
/home/test/opencv_contrib-3.1.0/modules/dnn/3rdparty/protobuf/src/google/protobuf/stubs/platform_macros.h:61:2: error: #error Host architecture was not detected as supported by protobuf
#error Host architecture was not detected as supported by protobuf