Fix race (and potential deadlock) at marker threads initialization

Fix race (and potential deadlock) at marker threads initialization

* include/private/gc_priv.h (GC_wait_for_markers_init): New prototype.
* mark.c (GC_wait_for_markers_init): New function (if PARALLEL_MARK).
* pthread_support.c (GC_mark_thread): Notify start_mark_threads()
about completion of marker data initialization.
* win32_threads.c (GC_mark_thread): Likewise.
* pthread_support.c (start_mark_threads): Add assertion about
GC_fl_builder_count; call GC_wait_for_markers_init.
* win32_threads.c (start_mark_threads): Likewise.
* win32_threads.c (GC_mark_thread): Reformat code.