Allow defining the ANDROID_ABI and WAMR_BUILD_TARGET for Android building about wasm-micro-runtime

[daniel7grant]

I tried it and it seems to be working and producing valid libs on every platform. Thank you, feel free to merge it.

[wenyongh]

Hi, yes, it is not a good way to hardcode the configurations in the cmake file, I remember it was written that way due to the GitHub CI which only supports android 32-bit. I submitted PR #3239 to rewrite the cmake file and CI file, could you help have a view? Thanks.

[daniel7grant]

Thanks for your work, I tried and it seems good to me.

Simultaneously I started working on this problem (main...daniel7grant:wasm-micro-runtime:feature/android-build-options), but your CMakeLists.txt seems to be more useful. The only thing I would add from my branch is the documentation: to add a note in the platform-mini/README.md that the flags should be used instead of modifying the CMakeLists.txt file.

[wenyongh]

OK, I updated the document according to you branch and made small changes. Please check whether more need to be added or changed?

[daniel7grant]

I don't see anything else. Thank you for your help.

[wenyongh]

Welcome.

[daniel7grant]

One more thing that I just noticed: if the WAMR_BUILD_TARGET is not set (i.e. running without any -D-s), it fails to compile for me, because the CMAKE_SYSTEM_PROCESSOR is not set. It currently breaks the default example code. Probably it would be a good idea to keep the current behaviour and build for a default platform. Otherwise, there should be a better error message, for example "Build target cannot be determined, set WAMR_BUILD_TARGET."

[wenyongh]

Yes, I did some test, it is not easy to set WAMR_BUILD_TARGET according to the CMAKE_SYSTEM_PROCESSOR: CMAKE_SYSTEM_PROCESSOR is available only when project (iwasm) is added in the CMakeLists.txt, but if the latter is added before setting ANDROID_ABI and CMAKE_TOOLCHAIN_FILE, the c/c++ compiler will be set to GNU compiler instead of NDK's compiler, and compilation errors will be reported. So I updated the PR to enforce the developer to set WAMR_BUILD_TARGET explicitly, and use it to set ANDRDOI_ABI if it isn't passed from the command line, and then pass it android.toolchain.cmake, which checks ANDRDOI_ABI and sets CMAKE_ANDROID_ARCH_ABI accordingly:
https://android.googlesource.com/platform/ndk/+/master/build/cmake/android.toolchain.cmake#110

Please help review and try again, let's merge the PR if it works fine for you.