LoongArch: Add alternative runtime patching mechanism

mainline inclusion
from mainline-v6.2-rc1
commit 19e5eb15
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6BWFP


CVE: NA

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Introduce the "alternative" mechanism from ARM64 and x86 for LoongArch
to apply runtime patching. The main purpose of this patch is to provide
a framework. In future we can use this mechanism (i.e., the ALTERNATIVE
and ALTERNATIVE_2 macros) to optimize hotspot functions according to cpu
features.

Signed-off-by: Jun Yi <yijun@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Change-Id: I204372e8f1ce7cf3c7bc7de9e8f6f16dbf06239a