LoongArch: module: Use got/plt section indices for relocations

Instead of saving a pointer to the .got, .plt and .plt_idx sections to
apply {got,plt}-based relocations, save and use their section indices
instead.

The mod->arch.{core,init}.{got,plt} pointers were problematic for live-
patch because they pointed within temporary section headers (provided by
the module loader via info->sechdrs) that would be freed after module
load. Since livepatch modules may need to apply relocations post-module-
load (for example, to patch a module that is loaded later), using section
indices to offset into the section headers (instead of accessing them
through a saved pointer) allows livepatch modules on LoongArch to pass
in their own copy of the section headers to apply_relocate_add() to
apply delayed relocations.

The method used is same as commit c8ebf64e ("arm64/module: use plt
section indices for relocations").

Signed-off-by: Hongchen Zhang <zhanghongchen@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>