feat(backend/riscv32): 实现基础的编译器功能

- 完成 RV32IMA 指令集的代码生成
- 添加整数运算、分支、调用等基本指令支持
- 实现从 IR 到机器码的转换
- 添加简单的测试用例和测试框架

ccompiler/middleend/ir.h

@@ -57,22 +57,25 @@ typedef struct {
     vector_header(extern_funcs, ir_func_t*);
 } ir_prog_t;
 
+typedef enum ir_node_tag {
+    IR_NODE_NULL,
+    IR_NODE_CONST_INT,
+    IR_NODE_ALLOC,
+    IR_NODE_LOAD,
+    IR_NODE_STORE,
+    IR_NODE_GET_PTR,
+    IR_NODE_OP,
+    IR_NODE_BRANCH,
+    IR_NODE_JUMP,
+    IR_NODE_CALL,
+    IR_NODE_RET,
+} ir_node_tag_t;
+
 struct ir_node {
     const ir_type_t* type;
     const char* name;
     vector_header(used_by, ir_node_t*);
-    enum {
-        IR_NODE_CONST_INT,
-        IR_NODE_ALLOC,
-        IR_NODE_LOAD,
-        IR_NODE_STORE,
-        IR_NODE_GET_PTR,
-        IR_NODE_OP,
-        IR_NODE_BRANCH,
-        IR_NODE_JUMP,
-        IR_NODE_CALL,
-        IR_NODE_RET,
-    } tag;
+    ir_node_tag_t tag;
     union {
         struct {
             int32_t val;