Fix PARALLEL_MARK for Windows 7+

Fix PARALLEL_MARK for Windows 7+
(Apply commit 57cc049 from 'master' branch.)

* doc/README.macros (DONT_USE_SIGNALANDWAIT): Remove.
* win32_threads.c (DONT_USE_SIGNALANDWAIT, GC_mark_mutex_waitcnt,
SignalObjectAndWait_type, signalObjectAndWait_func): Likewise.
* win32_threads.c (GC_marker_cv, GC_mark_thread, start_mark_threads,
GC_mark_mutex_state, GC_acquire_mark_lock, GC_release_mark_lock,
GC_wait_marker, GC_notify_all_marker, GC_thr_init): Do not check
DONT_USE_SIGNALANDWAIT macro (assume it is always on as multi-core
marker based on NT SignalObjectAndWait is broken in Windows 7+ leading
to a deadlock sometimes because the function is no longer atomic).

Conflicts:

    doc/README.macros