x86: separate out setup_pcpu_4k() from setup_per_cpu_areas()

Impact: modularize percpu first chunk allocation

x86 is gonna have a few different strategies for the first chunk
allocation.  Modularize it by separating out the current allocation
mechanism into pcpu_alloc_bootmem() and setup_pcpu_4k().

Signed-off-by: Tejun Heo <tj@kernel.org>