x86: fixes to LAPIC probing

x86: fixes to LAPIC probing

* Fix (unsafe) assumption that X86_FEATURE_APIC resided in feature word 0.
* All 64bit processors have local APICs; drop the vendor check.
* Unconditionally probe MSR_IA32_APICBASE (safely, to fail more gracefully in
  broken situations) and avoid a redundant double rdmsr().
* Avoid repeatedly OR'ing APICBASE_ENABLE and DEFAULT_PHYS_BASE when
  attempting to reenable the LAPIC.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>