ia64: on UP percpu variables are not small memory model

Tony says:

| The CONFIG_SMP=n path in ia64 makes quite radical changes ... rather
| than putting all the per-cpu stuff into the top 64K of address space
| and providing a per-cpu TLB mapping for that range to a different
| physical address ... it just makes all the per-cpu stuff link as ordinary
| variables in .data.

the new generic percpu code got confused about this as PER_CPU_ATTRIBUTES
was defined even on UP, so it picked up that small memory model - which
was not possible to get linked. The right fix is to only define that
on SMP. This resolved the build failures in my cross-compiling environment.

also link these variables into the .percpu section even on UP - some
assembly code has offset dependencies. (such as GET_IA64_MCA_DATA() in
arch/ia64/kernel/mca_asm.S)

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Tony Luck <tony.luck@intel.com>