x86, apic: rename 'genapic' to 'apic'

Impact: cleanup

Now that all APIC code is consolidated there's nothing 'gen' about
apics anymore - so rename 'struct genapic' to 'struct apic'.

This shortens the code and is nicer to read as well.

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