x86/percpu: Move some percpu accessors around to reduce ifdeffery

Move some percpu accessors around, mainly to reduce ifdeffery
and improve readabilty by following dependencies between
accessors.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240520080951.121049-2-ubizjak@gmail.com