LoongArch: Use la.pcrel instead of la.abs when it's trivially possible

Let's start to kill la.abs in preparation for the subsequent support of
the PIE kernel.

BTW, Re-tab the indention in arch/loongarch/kernel/entry.S for alignment.

Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>