libffi is linked into _ctypes_test.so but not actually used there #151422

A minor build nit: we link libffi into _ctypes_test.so unnecessarily. Due to a bug in the Emscripten dynamic loader this was preventing _ctypes_test.so from being loaded at all until Emscripten 4.0.19.

Linked PRs

• gh-151422: Don't link libffi into _ctypes_test.so #151423
• [3.15] gh-151422: Don't link libffi into _ctypes_test.so (GH-151423) #151516
• [3.14] gh-151422: Don't link libffi into _ctypes_test.so (GH-151423) #151517