/usr/lib/gcc-cross/aarch64-linux-gnu/ is in libobjc-6-dev-arm64-cross 6.4.0-17ubuntu1cross1.
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/usr/lib/gcc-cross/aarch64-linux-gnu/6/
/usr/lib/gcc-cross/aarch64-linux-gnu/6/include/
/usr/lib/gcc-cross/aarch64-linux-gnu/6/include/objc/
/usr/lib/gcc-cross/aarch64-linux-gnu/6/include/objc/NXConstStr.h
/usr/lib/gcc-cross/aarch64-linux-gnu/6/include/objc/Object.h
/usr/lib/gcc-cross/aarch64-linux-gnu/6/include/objc/Protocol.h
/usr/lib/gcc-cross/aarch64-linux-gnu/6/include/objc/message.h
/usr/lib/gcc-cross/aarch64-linux-gnu/6/include/objc/objc-decls.h
/usr/lib/gcc-cross/aarch64-linux-gnu/6/include/objc/objc-exception.h
/usr/lib/gcc-cross/aarch64-linux-gnu/6/include/objc/objc-sync.h
/usr/lib/gcc-cross/aarch64-linux-gnu/6/include/objc/objc.h
/usr/lib/gcc-cross/aarch64-linux-gnu/6/include/objc/runtime.h
/usr/lib/gcc-cross/aarch64-linux-gnu/6/include/objc/thr.h
/usr/lib/gcc-cross/aarch64-linux-gnu/6/libobjc.a
/usr/lib/gcc-cross/aarch64-linux-gnu/6/libobjc.so