percpu: Unify input section names

The two percpu helper macros have the section names duplicated.  So create
a new define to merge the two.  This also allows arches who need to link
things more directly themselves to avoid duplicating the input sections in
their linker script.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Tejun Heo <tj@kernel.org>