x86: Handle Extended APIC Register Space better

Extended APIC Register Space is AMD feature which was initially supported
in Jailhouse with a quick hack. It's fixed with this patch: now, the presence
of Extended APIC Register Space is determined as per APMv2, Sect. 16.3.4, and
the number of Extended Local Vector Tables is also determined in runtime.

As all of these is now done inside apic.c, and there is no need to keep
apic_reserved_bits[] public anymore.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
x86: Comments for apic_clear()

Added some comments for main steps that apic_clear() function
performs to restore APIC to the reset state.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>
x86: Remove struct farptr

This structure was previously used to reload CS register. As we switched
to the new switching method now, it is no longer needed.

Signed-off-by: Valentine Sinitsyn <valentine.sinitsyn@gmail.com>