ARM: spectre-v2: turn off the mitigation via boot cmdline param

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8SE4Y



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We enable spectre mitigation by default for ARM32, which may
cause performance regression. To offer an option to turn off
this feature, implement a cmdline parameter 'nospectre_v2' compatible
with mainline, which sets up a switch to skip invalidating BTB/icache
for A9/A15 in context switching and user abort.

Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
Signed-off-by: Yi Yang <yiyang13@huawei.com>