arm64: Provide a command line to disable spectre_bhb mitigation

hulk inclusion
category: feature
bugzilla: 186682, https://gitee.com/openeuler/kernel/issues/I5ZUU3



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openEuler 5.10 enables spectre-bhb mitigation by default for ARM64, which may
cause performance regression. implement a cmdline parameter
'nospectre_bhb' to provide an option to disable spectre-bhb mitigation.

Signed-off-by: Chen Jiahao <chenjiahao16@huawei.com>
Reviewed-by: Zhang Jianhua <chris.zjh@huawei.com>
Reviewed-by: Liao Chang <liaochang1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>