hw/loongarch: Add support loongson3 virt machine type.

Emulate a 3A5000 board use the new loongarch instruction.
3A5000 belongs to the Loongson3 series processors.
The board consists of a 3A5000 cpu model and the virt
bridge. The host 3A5000 board is really complicated and
contains many functions.Now for the tcg softmmu mode
only part functions are emulated.

More detailed info you can see
https://github.com/loongson/LoongArch-Documentation

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: <20220606124333.2060567-31-yangxiaojuan@loongson.cn>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

configs/devices/loongarch64-softmmu/default.mak

@@ -0,0 +1,3 @@
+# Default configuration for loongarch64-softmmu
+
+CONFIG_LOONGARCH_VIRT=y

configs/targets/loongarch64-softmmu.mak

@@ -0,0 +1,3 @@
+TARGET_ARCH=loongarch64
+TARGET_BASE_ARCH=loongarch
+TARGET_SUPPORTS_MTTCG=y