cpuidle: haltpoll: Only check boot_option_idle_override in x86

hulk inclusion
category: feature
bugzilla: 47727
CVE: NA

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boot_option_idle_override is defined only in x86/ia64.  Since haltpoll
supports x86 and arm64, let's check boot_option_idle_override only in
x86.

Signed-off-by: Peng Liang <liangpeng10@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>