Fix invalid initializer of CLOCK_TYPE variables if BSD_TIME

Fix invalid initializer of CLOCK_TYPE variables if BSD_TIME
(fix of commits 7e59301c1, 06ecaefca)

* alloc.c [!NO_CLOCK] (GC_start_time): Initialize to
CLOCK_TYPE_INITIALIZER (instead of 0).
* alloc.c [!NO_CLOCK] (GC_try_to_collect_inner, GC_stopped_mark,
GC_finish_collection): Initialize local variable of CLOCK_TYPE type to
CLOCK_TYPE_INITIALIZER (instead of 0); remove comment (duplicating that
in gc_priv.h).
* reclaim.c [!NO_CLOCK] (GC_reclaim_all): Likewise.
* include/private/gc_priv.h [!NO_CLOCK] (CLOCK_TYPE_INITIALIZER):
Define macro.