macOS: Don't inspect dead threadtls during exception handling. (#50871)
Otherwise we may end up with the state of a dead thread, as the system
IDs can alias. This can lead to an early return from the exception
handler, resulting in e.g. safepoint exceptions being actually delivered
to user code.
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Co-authored-by: Jameson Nash <vtjnash@gmail.com>
Co-authored-by: Cody Tapscott <topolarity@tapscott.me>