Don't link libdl when cross-compiling (pytorch#20522)

### Summary
The top-level CMakeLists.txt links libdl into executorch_core when
EXECUTORCH_USE_DL is on (the default):

    find_library(DL_LIBRARY_EXISTS NAMES dl)
if(DL_LIBRARY_EXISTS): target_link_libraries(executorch_core PRIVATE dl)

When cross-compiling for a bare-metal target (arm-none-eabi, esp,
zephyr), find_library still resolves the *host* libdl, so -ldl is linked
into executorch_core and propagates through the exported target
interface, failing the bare-metal executor-runner link ("cannot find
-ldl"). dladdr() (the only reason for the link) is compiled out on those
targets anyway (it's gated by the ET_USE_LIBDL macro, which the CMake
build never defines).

Two layers:
- Guard the link with `NOT CMAKE_CROSSCOMPILING` in CMakeLists.txt, so
any cross-compiled target is covered even without a preset.
- Set EXECUTORCH_USE_DL OFF explicitly in the arm_baremetal /
esp_baremetal / zephyr presets, documenting the intent for those
bare-metal builds.

### Test plan
Confirmed against the cortex-m FVP runner: with -ldl removed the
arm_executor_runner links and runs on Corstone-300.

Co-authored-by: Claude <noreply@anthropic.com>

Author: rascani

CMakeLists.txt

@@ -519,7 +519,11 @@ target_link_libraries(executorch_core PRIVATE program_schema)
 if(ANDROID)
   target_link_libraries(executorch_core PUBLIC log)
 endif()
-if(EXECUTORCH_USE_DL)
+# Skip when cross-compiling: find_library() resolves the host libdl even for a
+# bare-metal target (e.g. arm-none-eabi), which then links -ldl into the
+# baremetal runner and fails ("cannot find -ldl"). dladdr() isn't used on those
+# targets anyway. Per-target presets also set EXECUTORCH_USE_DL OFF explicitly.
+if(EXECUTORCH_USE_DL AND NOT CMAKE_CROSSCOMPILING)
   # Check if dl exists for this toolchain and only then link it.
   find_library(DL_LIBRARY_EXISTS NAMES dl)
   # Check if the library was found

tools/cmake/preset/arm_baremetal.cmake

@@ -29,6 +29,8 @@ set_overridable_option(EXECUTORCH_BUILD_EXECUTOR_RUNNER OFF)
 set_overridable_option(EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR OFF)
 set_overridable_option(EXECUTORCH_BUILD_EXTENSION_DATA_LOADER OFF)
 set_overridable_option(EXECUTORCH_BUILD_ARM_BAREMETAL ON)
+# Bare-metal has no libdl (see CMakeLists.txt EXECUTORCH_USE_DL block).
+set_overridable_option(EXECUTORCH_USE_DL OFF)
 set_overridable_option(EXECUTORCH_BUILD_KERNELS_QUANTIZED ON)
 set_overridable_option(EXECUTORCH_BUILD_EXTENSION_RUNNER_UTIL ON)
 set_overridable_option(EXECUTORCH_BUILD_CORTEX_M ON)

tools/cmake/preset/esp_baremetal.cmake

@@ -7,6 +7,8 @@ set(CMAKE_INSTALL_PREFIX "${CMAKE_BINARY_DIR}")
 set_overridable_option(EXECUTORCH_BUILD_EXECUTOR_RUNNER OFF)
 set_overridable_option(EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR OFF)
 set_overridable_option(EXECUTORCH_BUILD_EXTENSION_DATA_LOADER OFF)
+# Bare-metal has no libdl (see CMakeLists.txt EXECUTORCH_USE_DL block).
+set_overridable_option(EXECUTORCH_USE_DL OFF)
 set_overridable_option(EXECUTORCH_BUILD_KERNELS_QUANTIZED ON)
 set_overridable_option(EXECUTORCH_BUILD_EXTENSION_RUNNER_UTIL ON)
 set_overridable_option(EXECUTORCH_ENABLE_LOGGING ON)

tools/cmake/preset/zephyr.cmake

@@ -6,6 +6,8 @@
 
 set_overridable_option(EXECUTORCH_BUILD_COREML OFF)
 set_overridable_option(EXECUTORCH_ENABLE_EVENT_TRACER OFF)
+# Bare-metal has no libdl (see CMakeLists.txt EXECUTORCH_USE_DL block).
+set_overridable_option(EXECUTORCH_USE_DL OFF)
 set_overridable_option(EXECUTORCH_BUILD_KERNELS_LLM OFF)
 set_overridable_option(EXECUTORCH_BUILD_KERNELS_LLM_AOT OFF)
 set_overridable_option(EXECUTORCH_BUILD_EXTENSION_DATA_LOADER OFF)