posix: pthread: report appropriate return value instead of 0

Discovered this while implementing c11 threads, but there
was a regression recently that made it so that `pthread_join()`
would report success when attempting to join a thread that had
been detached with `pthread_detach()`.

Technically now that is undefined behaviour, but historically,
we have reported `EINVAL`, which was the older specified
return value.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>