cmake: add shared portable ops target and helper

Create libexecutorch_portable_ops.so under EXECUTORCH_BUILD_SHARED and
add reusable CMake helper logic used to construct shared wrapper targets.

Author: tomeuv

kernels/portable/CMakeLists.txt

@@ -106,6 +106,12 @@ install(
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/executorch/kernels/portable/
 )
 
+if(EXECUTORCH_BUILD_SHARED)
+  executorch_add_shared_library(
+    executorch_portable_ops portable_ops_lib portable_kernels executorch_shared
+  )
+endif()
+
 # Build the portable custom ops AOT library for registering custom ops into
 # PyTorch. Requires find_package(Torch), which must be called at root scope
 # before this subdirectory is processed. Not targeting ARM_BAREMETAL as aot_lib

tools/cmake/Utils.cmake

@@ -211,3 +211,29 @@ function(executorch_target_copy_mlx_metallib target)
     endif()
   endif()
 endfunction()
+
+# Create and install a shared library composed from dependency libraries. The
+# target links the provided dependencies and carries VERSION/SOVERSION.
+function(executorch_add_shared_library target_name)
+  set(_empty_source_name "${target_name}_empty.cpp")
+  file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/${_empty_source_name}"
+       "// intentionally empty"
+  )
+  add_library(
+    ${target_name} SHARED "${CMAKE_CURRENT_BINARY_DIR}/${_empty_source_name}"
+  )
+  target_link_libraries(${target_name} PRIVATE ${ARGN})
+  set_target_properties(
+    ${target_name}
+    PROPERTIES VERSION "${PROJECT_VERSION}"
+               SOVERSION "${PROJECT_VERSION_MAJOR}"
+               LINKER_LANGUAGE CXX
+  )
+  install(
+    TARGETS ${target_name}
+    EXPORT ExecuTorchTargets
+    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+    RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+  )
+endfunction()