Check thread is alive on each SuspendThread loop iteration (Win32)

Check thread is alive on each SuspendThread loop iteration (Win32)
(fix of commit 449eda034)

* win32_threads.c [!GC_NO_THREADS_DISCOVERY]
(GC_delete_gc_thread_no_free): Reset suspended if GC_win32_dll_threads.
* win32_threads.c [!MSWINCE] (GC_suspend): Move comment near exitCode
declaration to be near GetExitCodeThread() call; adjust comment.
* win32_threads.c [!MSWINCE && RETRY_GET_THREAD_CONTEXT] (GC_suspend):
Call GetExitCodeThread() on each iteration of the suspend loop; refine
ABORT message on ResumeThread failure.