docs/LoongArch: Update the links of ABI

The current links of ABI can not be found for some time, let us fix
the broken links.

By the way, the latest and official ABI documentation releases are
available at https://github.com/loongson/la-abi-specs

, but there are
no Chinese and pdf versions for now, so just do the minimal changes
to update the links so that they can be found, hope there are stable
links in the future.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>