x86/percpu: Declare const_pcpu_hot as extern const variable

const_pcpu_hot is aliased by linker to pcpu_hot, so there is no need
to use the DECLARE_PER_CPU_ALIGNED() macro. Also, declare const_pcpu_hot
as extern to avoid allocating storage space for the aliased structure.

Fixes: ed2f752e ("x86/percpu: Introduce const-qualified const_pcpu_hot to micro-optimize code generation")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20231130162949.83518-1-ubizjak@gmail.com
Closes: https://lore.kernel.org/oe-kbuild-all/202311302257.tSFtZnly-lkp@intel.com/