gc: extend optimized mode to strings (#10431)

vlib/builtin/array_notd_gcboehm_opt.v

@@ -0,0 +1,10 @@
+// dummy placeholder for functions from `array_d_gcboehm_opt.v`
+// that might be needed for compile time
+// `$if gcboehm_opt ? { ... } $else { ... }`
+
+module builtin
+
+// this is needed in `string.v`
+fn __new_array_noscan(mylen int, cap int, elm_size int) array {
+	return array{}
+}