selftests/seccomp: Add LoongArch selftesting support

BPF for LoongArch is supported now, add the selftesting support in
seccomp_bpf.c.

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