[vm, ffi] Emit the FFI callback stub as a raw blob and place it in the Fuchsia package.

Any blob in the package can be loaded as an executable VMO.

TEST=child CL
Bug: https://dartbug.com/52579
Change-Id: I61f7a84453da4da4733c5a70d4d19f0a6652581b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/496661
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

runtime/vm/dart_api_impl.cc

@@ -6911,6 +6911,28 @@ Dart_CreateVMAOTSnapshotAsAssembly(Dart_StreamingWriteCallback callback,
 #endif
 }
 
+DART_EXPORT Dart_Handle
+Dart_WriteCallbackStub(Dart_StreamingWriteCallback callback,
+                       void* callback_data) {
+#if defined(TARGET_ARCH_IA32)
+  return Api::NewError("AOT compilation is not supported on IA32.");
+#elif !defined(DART_PRECOMPILER)
+  return Api::NewError(
+      "This VM was built without support for AOT compilation.");
+#else
+  DARTSCOPE(Thread::Current());
+  API_TIMELINE_DURATION(T);
+  CHECK_NULL(callback);
+
+  callback(callback_data,
+           reinterpret_cast<uint8_t*>(
+               StubCode::FfiCallbackTrampoline().EntryPoint()),
+           StubCode::FfiCallbackTrampoline().Size());
+
+  return Api::Success();
+#endif
+}
+
 DART_EXPORT Dart_Handle
 Dart_CreateAppAOTSnapshotAsElf(Dart_StreamingWriteCallback callback,
                                void* callback_data,