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.

lib/common/threading.c

@@ -37,7 +37,7 @@ int g_ZSTD_threading_useless_symbol;
 static unsigned __stdcall worker(void *arg)
 {
     ZSTD_pthread_t* const thread = (ZSTD_pthread_t*) arg;
-    thread->arg = thread->start_routine(thread->arg);
+    thread->start_routine(thread->arg);
     return 0;
 }
 
@@ -55,7 +55,7 @@ int ZSTD_pthread_create(ZSTD_pthread_t* thread, const void* unused,
         return 0;
 }
 
-int ZSTD_pthread_join(ZSTD_pthread_t thread, void **value_ptr)
+int ZSTD_pthread_join(ZSTD_pthread_t thread)
 {
     DWORD result;
 
@@ -66,7 +66,6 @@ int ZSTD_pthread_join(ZSTD_pthread_t thread, void **value_ptr)
 
     switch (result) {
     case WAIT_OBJECT_0:
-        if (value_ptr) *value_ptr = thread.arg;
         return 0;
     case WAIT_ABANDONED:
         return EINVAL;