x86: clean up apic->apic_id_registered() methods

Impact: cleanup

x86 subarchitectures each defined a "apic_id_registered()" method,
which could be an inline function depending on which subarch we build
for, and which was also the name of a genapic field.

Untangle this namespace spaghetti by giving each of the instances
a separate name.

Also remove wrapper macro obfuscation.

Signed-off-by: Ingo Molnar <mingo@elte.hu>