cmake: build shared libexecutorch_portable_ops when EXECUTORCH_BUILD_SHARED=ON

Create a single libexecutorch_portable_ops.so that bundles
portable_ops_lib and portable_kernels, allowing consumers to link
portable op implementations via a shared library.

portable_ops_lib already has --whole-archive as an INTERFACE link
option from Codegen.cmake, so we do not wrap it in
$<LINK_LIBRARY:WHOLE_ARCHIVE,...> to avoid pulling the registration
constructor in twice. Link against executorch_shared (not
executorch_core) to share a single operator registry.

Author: tomeuv

kernels/portable/CMakeLists.txt

@@ -106,6 +106,38 @@ install(
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/executorch/kernels/portable/
 )
 
+# When building shared libraries, create a single
+# libexecutorch_portable_ops.so that bundles portable_ops_lib and
+# portable_kernels so consumers can link a shared library instead of
+# needing --whole-archive on the static archives.
+if(EXECUTORCH_BUILD_SHARED)
+  file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/portable_ops_shared_empty.cpp
+       "// intentionally empty"
+  )
+  add_library(
+    executorch_portable_ops SHARED
+    ${CMAKE_CURRENT_BINARY_DIR}/portable_ops_shared_empty.cpp
+  )
+  # portable_ops_lib already has --whole-archive as an INTERFACE link option
+  # (from executorch_target_link_options_shared_lib in Codegen.cmake), so do
+  # NOT wrap it in $<LINK_LIBRARY:WHOLE_ARCHIVE,...> to avoid pulling the
+  # registration constructor in twice.
+  target_link_libraries(
+    executorch_portable_ops
+    PRIVATE portable_ops_lib
+            portable_kernels
+            executorch_shared
+  )
+  set_target_properties(
+    executorch_portable_ops PROPERTIES LINKER_LANGUAGE CXX
+  )
+  install(
+    TARGETS executorch_portable_ops
+    EXPORT ExecuTorchTargets
+    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+  )
+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