feat(sem)补充符号表对库函数stoptime的支持

src/sem/SymbolTable.cpp

@@ -300,4 +300,14 @@ void SymbolTable::registerBuiltinFunctions() {
     starttime.scope_level = 0;
     starttime.is_builtin = true;
     addSymbol(starttime);
+
+    // stoptime: void stoptime()
+    Symbol stoptime;
+    stoptime.name = "stoptime";
+    stoptime.kind = SymbolKind::Function;
+    stoptime.type = ir::Type::GetFunctionType(ir::Type::GetVoidType(), {});  // 无参数，返回 void
+    stoptime.param_types = {};
+    stoptime.scope_level = 0;
+    stoptime.is_builtin = true;
+    addSymbol(stoptime);
 }