cgen: fix codegen for comptime multiline attr (fix #23964) (#24087)

vlib/v/gen/c/comptime.v

@@ -987,7 +987,8 @@ fn (mut g Gen) comptime_for(node ast.ComptimeFor) {
 				g.writeln('/* attribute ${i} */ {')
 				g.writeln('\t${node.val_var}.name = _SLIT("${attr.name}");')
 				g.writeln('\t${node.val_var}.has_arg = ${attr.has_arg};')
-				g.writeln('\t${node.val_var}.arg = _SLIT("${attr.arg}");')
+				g.writeln('\t${node.val_var}.arg = _SLIT("${util.smart_quote(attr.arg,
+					false)}");')
 				g.writeln('\t${node.val_var}.kind = AttributeKind__${attr.kind};')
 				g.stmts(node.stmts)
 				g.writeln('}')

vlib/v/tests/comptime/comptime_multiline_attr_test.v

@@ -0,0 +1,11 @@
+module main
+
+@[footer: 'Hello
+World']
+pub struct Config {}
+
+fn test_main() {
+	$for a in Config.attributes {
+		assert a.arg == 'Hello\nWorld'
+	}
+}