Fix assertion violation in GC_wait_builder called from start_mark_threads

Fix assertion violation in GC_wait_builder called from start_mark_threads
(fix commit 0ca6d3f)

* include/private/gc_priv.h [PARALLEL_MARK]
(GC_start_mark_threads_inner): Declare even if no CAN_HANDLE_FORK.
* misc.c [THREADS] (GC_start_mark_threads): Define also for the case
of PARALLEL_MARK and CAN_HANDLE_FORK - call GC_start_mark_threads_inner
surrounded with DISABLE/RESTORE_CANCEL.
* pthread_support.c [PARALLEL_MARK] (GC_start_mark_threads_inner):
Always define as GC_INNER.
* win32_threads.c [GC_PTHREADS_PARAMARK] (GC_start_mark_threads_inner):
Likewise.