[linux-arm64] bump up the version of gcc to 8
[linux-arm64] bump up the version of gcc to 8

linux-arm64 is currently using gcc 4, so it's difficult to take
advantage of modern C++. This PR bump up the version of gcc to
allow us to enjoy all the benefits of modern C++. (This PR is based
on #310)

dockcross/dockcross,dockcross/dockcross,thewtex/cross-compilers,thewtex/cross-compilers,thewtex/cross-compilers,dockcross/dockcross,dockcross/dockcross,thewtex/cross-compilers