Workaround 'if condition is the same as the previous one' cppcheck warnings

Workaround 'if condition is the same as the previous one' cppcheck warnings

* alloc.c [THREAD_LOCAL_ALLOC] (GC_stopped_mark): Set GC_world_stopped
to FALSE after GC_on_collection_event(GC_EVENT_PRE_START_WORLD).
* alloc.c [THREAD_LOCAL_ALLOC] (GC_stopped_mark): Collapse adjacent
"if" statements.
* mark.c [!GC_DISABLE_INCREMENTAL] (GC_initiate_gc): Likewise.
* pthread_stop_world.c [!NACL && !GC_OPENBSD_UTHREADS
&& !GC_NETBSD_THREADS_WORKAROUND] (GC_start_world): Likewise.