Add emscripten guards to shared_library.c's dlopen path (ros2#591) (ros2#599)

* fix: add emscripten guards to shared_library.c's dlopen path

Two related gaps in rcutils_load_shared_library() / rcutils_get_platform_library_name(),
found while getting rcl_logging_implementation's dlopen-by-name backend
selection working on emscripten-wasm32:

- rcutils_get_platform_library_name() had no emscripten branch, so it always
  fell through with written == 0 ("failed to format library name"),
  regardless of which RCL_LOGGING_IMPLEMENTATION backend was requested.
  wasm32 side modules use the same "lib<name>.so" convention as Linux.

- The post-dlopen path-resolution code took the _GNU_SOURCE branch (which
  emscripten's headers define) and called dlinfo(..., RTLD_DI_LINKMAP, ...).
  Emscripten's dlopen()/dlinfo() are a JS-backed shim, not glibc's, and
  don't support reading back a real struct link_map -- so a successful
  dlopen() got treated as a failure once dlinfo() returned -1. The #else
  branch (reuse the path dlopen() was given) already covers this platform
  correctly.

Verified end-to-end on a ROS 2 rolling + rmw_zenoh_pico + real-pthreads
emscripten-wasm32 build: a wasm32 rclpy talker's logging initialization
(rcl_logging_configure -> rcl_logging_implementation -> this dlopen path)
now succeeds instead of aborting with "failed to load any logging
implementations".

(cherry picked from commit a2d8f50)

Signed-off-by: Alejandro Hernandez Cordero <ahcorde@gmail.com>
Co-authored-by: Tobias Fischer <info@tobiasfischer.info>
Co-authored-by: Alejandro Hernandez Cordero <ahcorde@gmail.com>

Author: 3 people

src/shared_library.c

@@ -124,7 +124,14 @@ rcutils_load_shared_library(
     goto fail;
   }
   lib->library_path = rcutils_strdup(image_name, lib->allocator);
-#elif defined(_GNU_SOURCE) && !defined(__QNXNTO__) && !defined(__ANDROID__) && !defined(__OHOS__)
+  // Emscripten defines _GNU_SOURCE but its dlopen()/dlinfo() are a JS-backed
+  // shim, not glibc's -- RTLD_DI_LINKMAP support (reading back a real
+  // struct link_map) doesn't exist there. A successful dlopen() would
+  // otherwise get treated as a failure once dlinfo() returns -1 below; the
+  // #else branch already covers this correctly (it just reuses the path we
+  // opened the library from, no introspection needed).
+#elif defined(_GNU_SOURCE) && !defined(__QNXNTO__) && !defined(__ANDROID__) && \
+  !defined(__OHOS__) && !defined(__EMSCRIPTEN__)
   struct link_map * map = NULL;
   if (dlinfo(lib->lib_pointer, RTLD_DI_LINKMAP, &map) != 0) {
     RCUTILS_SET_ERROR_MSG_WITH_FORMAT_STRING("dlinfo error: %s", dlerror());
@@ -291,7 +298,12 @@ rcutils_get_platform_library_name(
 
   int written = 0;
 
-#if defined(__linux__) || defined(__QNXNTO__)
+  // rcl_logging_implementation dlopens its backend (spdlog or noop) by name
+  // at runtime via this function -- with no emscripten case it always falls
+  // through with written == 0 ("failed to format library name"), regardless
+  // of which backend RCL_LOGGING_IMPLEMENTATION selects. wasm32 side modules
+  // use the same "lib<name>.so" naming convention as Linux.
+#if defined(__linux__) || defined(__QNXNTO__) || defined(__EMSCRIPTEN__)
   if (debug) {
     if (buffer_size >= (strlen(library_name) + 8)) {
       written = rcutils_snprintf(