[asavah]

I edited my post, check the remark about the CMAKE_TOOLCHAIN_FILE

Edit:
Hmm in my buildsystem it detects both compilers right

host:

bash:
flatbuffers: configuring ...

[21:29:58] [DEBUG]: kbs.console.kbsConsole.configure(): logging disabled via config file

-- The C compiler identification is GNU 7.3.0

-- The CXX compiler identification is GNU 7.3.0

-- Check for working C compiler: /usr/bin/gcc

-- Check for working C compiler: /usr/bin/gcc -- works

-- Detecting C compiler ABI info

-- Detecting C compiler ABI info - done

-- Detecting C compile features

-- Detecting C compile features - done

-- Check for working CXX compiler: /usr/bin/g++

-- Check for working CXX compiler: /usr/bin/g++ -- works

-- Detecting CXX compiler ABI info

-- Detecting CXX compiler ABI info - done

-- Detecting CXX compile features

-- Detecting CXX compile features - done

-- Configuring done

-- Generating done

-- Build files have been written to: /home/asavah/kross/tmp/rock64/flatbuffers

bash:
target:

​​​​​​​flatbuffers: configuring ...

[21:30:26] [DEBUG]: kbs.console.kbsConsole.configure(): logging disabled via config file

-- The C compiler identification is GNU 8.2.0

-- The CXX compiler identification is GNU 8.2.0

-- Check for working C compiler: /home/asavah/kross/host/rock64/bin/aarch64-rock64-linux-gnu-gcc

-- Check for working C compiler: /home/asavah/kross/host/rock64/bin/aarch64-rock64-linux-gnu-gcc -- works

-- Detecting C compiler ABI info

-- Detecting C compiler ABI info - done

-- Detecting C compile features

-- Detecting C compile features - done

-- Check for working CXX compiler: /home/asavah/kross/host/rock64/bin/aarch64-rock64-linux-gnu-g++

-- Check for working CXX compiler: /home/asavah/kross/host/rock64/bin/aarch64-rock64-linux-gnu-g++ -- works

-- Detecting CXX compiler ABI info

-- Detecting CXX compiler ABI info - done

-- Detecting CXX compile features

-- Detecting CXX compile features - done

-- Configuring done

-- Generating done

-- Build files have been written to: /home/asavah/kross/tmp/rock64/flatbuffers