genirq: add generic I-pipe core
This commit provides the arch-independent bits for implementing the interrupt pipeline core, a lightweight layer introducing a separate, high-priority execution stage for handling all IRQs in pseudo-NMI mode, which cannot be delayed by the regular kernel code. See Documentation/ipipe.rst for details about interrupt pipelining. Architectures which support interrupt pipelining should select HAVE_IPIPE_SUPPORT, along with implementing the required arch-specific code. In such a case, CONFIG_IPIPE becomes available to the user via the Kconfig interface for enabling the feature.
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