x86/apic: KVM: Move lapic set/clear_vector() helpers to common code

Move apic_clear_vector() and apic_set_vector() helper functions to
apic.h in order to reuse them in the Secure AVIC guest APIC driver
in later patches to atomically set/clear vectors in the APIC backing
page.

No functional change intended.

Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Acked-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20250709033242.267892-13-Neeraj.Upadhyay@amd.com


Signed-off-by: Sean Christopherson <seanjc@google.com>