Fixes two bugs in the Windows thread / pthread translation layer

1. If threads are resized the threads' `ZSTD_pthread_t` might move
while the worker still holds a pointer into it (see more details in #3120).
2. The join operation was waiting for a thread and then return its `thread.arg`
as a return value, but since the `ZSTD_pthread_t thread` was passed by value it
would have a stale `arg` that wouldn't match the thread's actual return value.

This fix changes the `ZSTD_pthread_join` API and removes support for returning
a value. This means that we are diverging from the `pthread_join` API and this
is no longer just an alias.
In the future, if needed, we could return a Windows thread's return value using
`GetExitCodeThread`, but as this path wouldn't be excised in any case, it's
preferable to not add it right now.

tests/fuzzer.c

@@ -431,8 +431,8 @@ static int threadPoolTests(void) {
 
         ZSTD_pthread_create(&t1, NULL, threadPoolTests_compressionJob, &p1);
         ZSTD_pthread_create(&t2, NULL, threadPoolTests_compressionJob, &p2);
-        ZSTD_pthread_join(t1, NULL);
-        ZSTD_pthread_join(t2, NULL);
+        ZSTD_pthread_join(t1);
+        ZSTD_pthread_join(t2);
 
         assert(!memcmp(decodedBuffer, decodedBuffer2, CNBuffSize));
         free(decodedBuffer2);