x86/apic: Annotate global config variables as "read-only after init"

Mark the APIC's global config variables that are constant after boot as
__ro_after_init to help document that the majority of the APIC config is
not changed at runtime, and to harden the kernel a smidge.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190805212134.12001-1-sean.j.christopherson@intel.com