Fix data race in mark_thread when updating mark_no

Fix data race in mark_thread when updating mark_no
(Cherry-pick commit 266c2891 from 'release-7_6' branch.)

* mark.c [PARALLEL_MARK] (GC_mark_local, GC_help_marker): Update
description comment; remove GC_acquire_mark_lock() call at the function
beginning; remove GC_release_mark_lock() call on return.
* mark.c [PARALLEL_MARK] (GC_do_parallel_mark): Remove
GC_acquire/release_mark_lock() calls around GC_mark_local() call.
* pthread_support.c [PARALLEL_MARK] (GC_mark_thread): Remove
GC_release_mark_lock() call.
* win32_threads.c [PARALLEL_MARK] (GC_mark_thread): Likewise.