LoongArch: Consolidate __ex_table construction

Consolidate all the __ex_table constuction code with a _ASM_EXTABLE or
_asm_extable helper.

There should be no functional change as a result of this patch.

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>