x86/kvm/vmx: Fix x86_64 CFI build

x86/kvm/vmx: Fix x86_64 CFI build

It was missed that idt_do_interrupt_irqoff() gets compiled on x84_64;
this is a problem for CFI builds because it includes an unadorned
indirect call. It is however completely dead code.

Rework things to not emit this function at all.

Fixes: 0701c9e17bd9 ("x86/kvm/vmx: Move IRQ/NMI dispatch from KVM into x86 core")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: Calvin Owens <calvin@wbinvd.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260526090631.GA4149641@noisy.programming.kicks-ass.net