Makes fiber_entryPoint() noreturn

[denizzzka]

This makes fiber trampoline functions unnecessary? I.e., will this clear the "link register" prior to calling fiber_entryPoint?

upd: Most likely not, because this added assertion still needs to be catched somehow

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[denizzzka]

fiber.d contains comment:

 * FIXME: fiber_entrypoint might benefit from a @noreturn attribute, but D doesn't have one.