x86: Simplify smpboot_alloc by merging x86-{32,64} code as far as

x86: Simplify smpboot_alloc by merging x86-{32,64} code as far as
possible.

We still need one ifdef, as x86-32 does not have a compat_gdt_table.

On x86-32 there is 1/2-page wastage due to allocating a whole page for
the per-CPU IDT, however we expect very few users of the x86-32
hypervisor. Those that cannot move to the 64-bit hypervisor are likely
using old single-processor systems or new single-procesor netbooks. On
UP and small MP systems, the wastage is insignificant.

Signed-off-by: Keir Fraser <keir@xen.org>
Committed-by: Keir Fraser <keir@xen.org>