x86/apic: Register boot CPU APIC early

Register the boot CPU APIC right when the boot CPUs APIC is read from the
hardware. No point is doing this on random places and having wild
heuristics to save the boot CPU APIC ID slot and CPU number 0 reserved.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Tested-by: Juergen Gross <jgross@suse.com> # Xen PV (dom0 and unpriv. guest)