Add Hello World demo

demo/hello_world/.gitignore

@@ -0,0 +1,2 @@
+hello_world
+occlum_workspace

demo/hello_world/Makefile

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+CC := musl-clang
+CFLAGS := -fPIC -pie
+
+.PHONY: all test test-native clean
+
+all: hello_world
+
+hello_world: hello_world.c
+	$(CC) $(CFLAGS) $^ -o $@
+
+# Run hello_world on Occlum inside an SGX enclave
+test:
+	rm -rf occlum_workspace
+	mkdir occlum_workspace
+	cd occlum_workspace && \
+		occlum init && \
+		cp ../hello_world image/bin && \
+		occlum build && \
+		occlum run /bin/hello_world
+
+# Run hello_world on the native OS (e.g., Linux)
+test-native:
+	./hello_world
+
+clean:
+	rm -rf hello_world occlum_workspace

demo/hello_world/hello_world.c

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+#include <stdio.h>
+
+int main() {
+    printf("Hello World\n");
+    return 0;
+}