Fix race condition in the Windows thread / pthread translation layer

When spawning a Windows thread we have small worker wrapper function that translates
between the interfaces of Windows and POSIX threads.
This wrapper is given a pointer that might get stale before the worker starts running,
resulting in UB and crashes.
This commit adds synchronization so that we know the wrapper has finished reading the data
it needs before we allow the main thread to resume execution.

lib/common/threading.h

@@ -61,11 +61,7 @@ extern "C" {
 #define ZSTD_pthread_cond_broadcast(a)  WakeAllConditionVariable((a))
 
 /* ZSTD_pthread_create() and ZSTD_pthread_join() */
-typedef struct {
-    HANDLE handle;
-    void* (*start_routine)(void*);
-    void* arg;
-} ZSTD_pthread_t;
+typedef HANDLE ZSTD_pthread_t;
 
 int ZSTD_pthread_create(ZSTD_pthread_t* thread, const void* unused,
                    void* (*start_routine) (void*), void* arg);