work around missing os:iphoneos on "Reapply "Depend on extension/threadpool, not thread_parallel_interface, in buck (#9511)""

Trying again with internal fixes. Original summary:

I planned to do this everywhere and forgot. Clean it all up, leave a
note, enforce the note with visibility. This makes sure everything in
buck-land gets ET_USE_THREADPOOL.

Differential Revision: [D72090889](https://our.internmc.facebook.com/intern/diff/D72090889/)

[ghstack-poisoned]

Author: Github Executorch

extension/threadpool/targets.bzl

@@ -47,7 +47,9 @@ def define_common_targets():
 
     runtime.cxx_library(
         name = "threadpool",
-        exported_deps = select({
+        # TODO: OSS doesn't have os:iphoneos. Sync buck2 prelude
+        # update to add it and remove duplication.
+        exported_deps = (select({
             # Major operating systems should be able to use threadpool.
             "ovr_config//os:linux": [":threadpool_lib"],
             "ovr_config//os:macos": [":threadpool_lib"],
@@ -58,7 +60,17 @@ def define_common_targets():
             "ovr_config//os:none": ["//executorch/runtime/kernel:thread_parallel_interface"],
             # If we don't know what it is, disable threadpool out of caution.
             "DEFAULT": ["//executorch/runtime/kernel:thread_parallel_interface"],
-        }),
+        }) if not runtime.is_oss else select({
+            # Major operating systems should be able to use threadpool.
+            "ovr_config//os:linux": [":threadpool_lib"],
+            "ovr_config//os:macos": [":threadpool_lib"],
+            "ovr_config//os:windows": [":threadpool_lib"],
+            "ovr_config//os:android": [":threadpool_lib"],
+            # Machines without an operating system shouldn't.
+            "ovr_config//os:none": ["//executorch/runtime/kernel:thread_parallel_interface"],
+            # If we don't know what it is, disable threadpool out of caution.
+            "DEFAULT": ["//executorch/runtime/kernel:thread_parallel_interface"],
+        })),
         visibility = [
             "//executorch/...",
             "@EXECUTORCH_CLIENTS",