Use g++-8 to keep consistency in each architecture

[hadusam]

What this patch does to fix the issue.

Close #1189

% docker run --rm -it blueoil_masuda:v0.27.0-18-g4db3ba40 /bin/bash
root@2cc8feb8e3a2:/home/blueoil# g++ --version
g++ (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

root@2cc8feb8e3a2:/home/blueoil# arm-linux-gnueabihf-g++ --version
arm-linux-gnueabihf-g++ (Ubuntu/Linaro 8.4.0-1ubuntu1~18.04) 8.4.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

root@2cc8feb8e3a2:/home/blueoil# aarch64-linux-gnu-g++ --version
aarch64-linux-gnu-g++ (Ubuntu/Linaro 8.4.0-1ubuntu1~18.04) 8.4.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Link to any relevant issues or pull requests.

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[iizukak]

OA
Sorry for review delaying

[iizukak]

/ready

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