Support for Raspberry Pi 3 compilation

[CorentinB]

Bonjour from France!

Following the instructions described in this issue, I implemented a compilation flag for Raspberry Pi 3 build.

It's simply -DPI3=ON, and need to be used with -DCMAKE_TOOLCHAIN_FILE=../pi3.toolchain.cmake to make the compilation possible.

I've only tested it on a Raspberry Pi 3B+ model, but maybe it can work on other models too.

Here are the benchmarks results on the Pi 3B+:

pi@raspberrypi:~/ncnn/benchmark $ ./benchncnn 8 4 0
loop_count = 8
num_threads = 4
powersave = 0
      squeezenet  min =  117.58  max =  127.02  avg =  118.90
       mobilenet  min =  214.80  max =  226.69  avg =  216.65
    mobilenet_v2  min =  228.65  max =  238.24  avg =  230.24
      shufflenet  min =   79.97  max =   80.61  avg =   80.29
       googlenet  min =  365.26  max =  407.66  avg =  380.22
        resnet18  min =  556.46  max =  588.56  avg =  573.58
         alexnet  min =  436.50  max =  503.09  avg =  468.44
           vgg16  min = 2765.14  max = 2893.23  avg = 2811.79
  squeezenet-ssd  min =  378.21  max =  405.30  avg =  388.32
   mobilenet-ssd  min =  391.99  max =  410.40  avg =  397.88
  mobilenet-yolo  min =  977.81  max = 1032.28  avg =  995.55

I think it can be a great addition, allowing makers all around the world to easily test and work with NCNN with a really small investment.

Have a nice day.

[tencent-adm]

All committers have signed the CLA.

[coveralls]

Coverage remained the same at ?% when pulling 698ffa4 on CorentinB:pi3 into 4ec1c17 on Tencent:master.

[nihui]

thanks a lot ^^:)