Install libexecutorch.a and libportable_kernels.a (#1036)

[larryliu0820]

Stack from ghstack (oldest at bottom):

• [cmake] Fix CI job #1114
• -> Install libexecutorch.a and libportable_kernels.a (#1036) #1113

Summary:
This PR is the first one to decentralize options only used by examples, from root level CMakeLists.txt to examples directories.

To install these two libraries (libexecutorch.a and libportable_kernels.a), user needs to run:

    cmake -DBUCK2="$BUCK" \
            -DCMAKE_INSTALL_PREFIX=cmake-out \
            -DPYTHON_EXECUTABLE="$PYTHON_EXECUTABLE" \
            -Bcmake-out .
    cmake --build cmake-out -j9 --target install

Then in the console output we will have:

-- Installing: /home/larryliu/executorch/cmake-out/lib/libportable_kernels.a
-- Installing: /home/larryliu/executorch/cmake-out/lib/libexecutorch.a
-- Installing: /home/larryliu/executorch/cmake-out/lib/cmake/ExecuTorch/ExecuTorchConfig.cmake

In order to find these libraries, we can do the following in example CMakeLists.txt:

find_package(ExecuTorch REQUIRED)

Then the example CMakeLists.txt can depend on executorch and portable_kernels.

Reviewed By: dbort

Differential Revision: D50482400

Pulled By: larryliu0820

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/1113

• 📄 Preview Python docs built from this PR

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❌ 2 New Failures

As of commit 375cbcb with merge base 7814dd7 ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
• pull / test-selective-build-linux (cmake) / linux-job (gh)

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