Fix GC_marker_Id elements initialization (WinCE)

Fix GC_marker_Id elements initialization (WinCE)

* win32_threads.c (GC_marker_cv, GC_marker_Id): Move variable
definition up to precede GC_mark_thread (only if PARALLEL_MARK and
DONT_USE_SIGNALANDWAIT).
* win32_threads.c (GC_mark_thread): Set corresponding element of
GC_marker_Id[] to GetCurrentThreadId() value (only if PARALLEL_MARK
and DONT_USE_SIGNALANDWAIT).
* win32_threads.c (start_mark_threads): Do not set GC_marker_Id[]
elements here because GetCurrentThreadId should be invoked from the
corresponding thread; update comment (only if PARALLEL_MARK and
DONT_USE_SIGNALANDWAIT).