mksysmap: Fix the mismatch of 'L0' symbols in System.map

When System.map was generated, the kernel used mksysmap to filter the
kernel symbols, we need to filter "L0" symbols in LoongArch architecture.

$ cat System.map | grep L0
9000000000221540 t L0

The L0 symbol exists in System.map, but not in .tmp_System.map. When
"cmp -s System.map .tmp_System.map" will show "Inconsistent kallsyms
data" error message in link-vmlinux.sh script.

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>