selftests/bpf: Add __arch_loongarch to limit test cases for LoongArch

selftests/bpf: Add __arch_loongarch to limit test cases for LoongArch

Make it possible to limit certain tests to LoongArch, just like it is
already done for x86_64, arm64, riscv64, and s390x.

This is a follow up patch of:

  commit ee7fe84468b1 ("selftests/bpf: __arch_* macro to limit test cases to specific archs")
  commit 1e4e6b9e260d ("selftests/bpf: Add __arch_s390x macro")

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