locking/atomic/x86: Redeclare x86_32 arch_atomic64_{add,sub}() as void

Correct the return type of x86_32 arch_atomic64_add() and
arch_atomic64_sub() functions to 'void' and remove redundant return.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240605181424.3228-2-ubizjak@gmail.com