Add concurrency checks to ExceptionHandling.RaiseAppDomainUnhandledExceptionEvent

[tommcdon]

This issue is tracking follow-up work from PR conversation here: #117832 (comment)

We want to prevent multiple AppContext.OnhandledException callbacks triggered via multi-threaded calls to ExceptionHandling.RaiseAppDomainUnhandledExceptionEvent

[tommcdon]

closing as dupe to #117840