LoongArch: Add la_abs macro implementation

Use the "la_abs macro" instead of the "la.abs pseudo instruction" to
prepare for the subsequent PIE kernel. When PIE is not enabled, la_abs
is equivalent to la.abs.

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