Rename and reorder profiling callback events (to match Mono GC API)

Rename and reorder profiling callback events (to match Mono GC API)

* include/gc.h (GCEventKind): Reorder events.
* include/gc.h (GCEventKind): Rename events (GC_EVENT_COLLECTION_BEGIN
to GC_EVENT_START, GC_EVENT_MARK_BEGIN to GC_EVENT_MARK_START,
GC_EVENT_COLLECTION_END to GC_EVENT_END,
GC_EVENT_STOPWORLD_BEGIN/END to GC_EVENT_PRE/POST_STOP_WORLD,
GC_EVENT_STARTWORLD_BEGIN/END to GC_EVENT_PRE/POST_START_WORLD).
* alloc.c (GC_try_to_collect_inner, start_world_inner, GC_stopped_mark,
GC_finish_collection): Likewise.