PCI: hv: Avoid the retarget interrupt hypercall in irq_unmask() on ARM64

PCI: hv: Avoid the retarget interrupt hypercall in irq_unmask() on ARM64

On ARM64 Hyper-V guests, SPIs are used for the interrupts of virtual PCI
devices, and SPIs can be managed directly via GICD registers. Therefore
the retarget interrupt hypercall is not needed on ARM64.

An arch-specific interface hv_arch_irq_unmask() is introduced to handle
the architecture level differences on this. For x86, the behavior
remains unchanged, while for ARM64 no hypercall is invoked when
unmasking an irq for virtual PCI devices.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Boqun Feng <[email protected]>
Signed-off-by: Lorenzo Pieralisi <[email protected]>
Reviewed-by: Michael Kelley <[email protected]>