1: fix cmake cross-ios failed caused by df118a87
   build static lib for APPLE define for XCODE third_party framework including require

2: megbrain_test/megdnn_test build when MGE_INFERENCE_ONLY=ON
   now u can build megbrain_test/megdnn_test by:
   EXTRA_CMAKE_ARGS="-DMGE_WITH_TEST=ON" ./scripts/cmake-build/xxx.sh
   example macos-cross-ios build megdnn_test for IOS by
   EXTRA_CMAKE_ARGS="-DMGE_WITH_TEST=ON" ./scripts/cmake-build/cross_build_ios_arm_inference.sh

3: reuse host flatc build when cross build mode
GitOrigin-RevId: 132f4bf893567bdb1d54de506449950513a5841f

GitOrigin-RevId: 19f3c33003

Also, upgrade to CMake 3.13.

The commit also contains significant refactors, as otherwise it is not
possible to properly export target `megengine` to
MegEngine-targets.cmake:

1. Optionally use system provided Flatbuffers.
2. Optionally use system provided MKL-DNN (Tested with Debian).
3. Refactor megbrain and megdnn targets into object libraries.
4. Set different path in BUILD_INTERFACE and INSTALL_INTERFACE of
   various target_include_directories.
5. Specify PUBLIC/PRIVATE on various target_link_libraries.

GitOrigin-RevId: df118a879e

GitOrigin-RevId: f0c8338beb