Avoid busy waiting in mark_thread while GC_parallel is false

Avoid busy waiting in mark_thread while GC_parallel is false
(Cherry-pick commit 46d2a44 from 'release-7_6' branch.)

* mark.c [PARALLEL_MARK] (GC_help_marker): Add assertion that
GC_parallel is true (i.e. GC_markers_m1 is non-zero).
* pthread_support.c [PARALLEL_MARK && CAN_HANDLE_FORK]
(start_mark_threads): Set GC_markers_m1 value before starting
the first marker thread (it is already set if fork handling is off).
* win32_threads.c [GC_PTHREADS_PARAMARK && CAN_HANDLE_FORK]
(start_mark_threads): Likewise.
* pthread_support.c [PARALLEL_MARK] (start_mark_threads):
Adjust GC_markers_m1 value only if pthread_create failed.
* win32_threads.c [GC_PTHREADS_PARAMARK] (start_mark_threads):
Likewise.