LoongArch: Add alternative runtime patching mechanism

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>