percpu: fix build breakage on s390 and cleanup build configuration tests

Commit bbddff05 (percpu: use percpu allocator on UP too) incorrectly
excluded pcpu_build_alloc_info() on SMP configurations which use
generic setup_per_cpu_area() like s390.  The config ifdefs are
becoming confusing.  Fix and clean it up by,

* Move pcpu_build_alloc_info() right on top of its two users -
  pcpu_{embed|page}_first_chunk() which are already in CONFIG_SMP
  block.

* Define BUILD_{EMBED|PAGE}_FIRST_CHUNK which indicate whether each
  first chunk function needs to be included and use them to control
  inclusion of the three functions to reduce confusion.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Sachin Sant <sachinp@in.ibm.com>