x86/percpu: Move raw_percpu_xchg_op() to a better place

Move raw_percpu_xchg_op() together with this_percpu_xchg_op()
and trivially rename some internal variables to harmonize them
between macro implementations.

No functional changes intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240320083127.493250-2-ubizjak@gmail.com