lib: sbi: Remove xchg/cmpxchg implemented via lr/sc

lr/sc is part of the A extension. If the A extension is not supported,
lr/sc cannot be used. So remove xchg/cmpxchg.

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>