Allow low level pthread APIs to be called from Wasm Workers.

As of today in pure Wasm Worker build its simply not possible to call pthread functions.

In hybrid Wasm Workers + pthreads builds the APIs are available but we get undefined behaviour if pthread APIs are called from a Wasm Worker (since pthread_self() returns NULL).

Even in the pure Wasm Worker builds its problematic that we cannot use low level pthread APIs. For example we recently have to patch libc++ to avoid the pthread_mutex API. See #26345

We recently had some discussion of allowing pthread APIs in Wasm Workers, provided it can be done at zero cost for programs that don't use these APIs: https://github.com/emscripten-core/emscripten/pull/26568#issuecomment-4158200448