linked event (can be nil), depends on event type: for GCStart: the GCStop for GCScanStart: the GCScanDone for GCSweepStart: the GCSweepDone for GoCreate: first GoStart of the created goroutine for GoStart: the associated GoEnd, GoBlock or other blocking event for GoSched/GoPreempt: the next GoStart for GoBlock and other blocking events: the unblock event for GoUnblock: the associated GoStart for blocking GoSysCall: the associated GoSysExit for GoSysExit: the next GoStart