Remove GC_win32_dll_threads checking inside pthread-specific code

Remove GC_win32_dll_threads checking inside pthread-specific code

* win32_threads.c (GC_unregister_my_thread): Remove check for
GC_PTHREADS inside GC_win32_dll_threads (since DllMain-based thread
registration is not supported for pthreads).
* win32_threads.c (GC_pthread_join, GC_pthread_create): Replace check
for non-GC_win32_dll_threads mode with the assertion; move local
variable declarations to function beginning.