tests/tcg/loongarch64: Add hello/memory test in loongarch64 system

- We write a very minimal softmmu harness.
- This is a very simple smoke test with no need to run a full Linux/kernel.
- The Makefile.softmmu-target record the rule to run.

Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220606124333.2060567-43-yangxiaojuan@loongson.cn>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>