include: Use more consistent name for atomic xchg() and cmpxchg()

We should remove the "arch_" prefix from atomic xchg() and cmpxchg()
function names to have consistent naming of all atomic functions.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>