[Add] import xv6-riscv and submodules

.gitmodules

@@ -0,0 +1,9 @@
+[submodule "loongarch64-linux-gnu"]
+	path = loongarch64-linux-gnu
+	url = https://gitee.com/loongarch_community/loongarch64-linux-gnu
+[submodule "nscscc-team-la32r"]
+	path = nscscc-team-la32r
+	url = https://gitee.com/loongson-edu/nscscc-team-la32r
+[submodule "lasoft/la32r-nemu"]
+	path = lasoft/la32r-nemu
+	url = https://gitee.com/wwt_panache/la32r-nemu

Makefile

@@ -0,0 +1,26 @@
+# prepare loongarch toolchain
+
+TOOLCHAIN_PK	:= loongarch64-linux-gnu/toolchain-loongarch64-linux-gnu-gcc8-host-x86_64-2022-07-18.tar.xz
+
+LA_PATH			:= ext/toolchain-loongarch64-linux-gnu-gcc8-host-x86_64-2022-07-18/bin
+LA_ARCH			:= loongarch64-linux-gnu-
+LA				:= $(LA_PATH)/$(LA_ARCH)
+
+GCC				:= $(LA)gcc
+OD				:= $(LA)objdump
+OC				:= $(LA)objcopy
+RE				:= $(LA)readelf
+
+LA_TOOLS		:= $(GCC) $(OD) $(OC) $(RE)
+
+$(TOOLCHAIN_PK):
+	git submodule update --init
+
+unpk-gcc: $(TOOLCHAIN_PK)
+	mkdir -p ext
+	tar -xvf $(TOOLCHAIN_PK) -C ext
+
+$(LA_TOOLS): unpk-gcc
+
+# prepare loongarch source code
+