py/persistentcode: Define mp_raw_code_save_file() for any unix target.

A unix target should provide POSIX open/write/close functions regardless of
its machine architecture.  Fixes issue #3325.

py/persistentcode.c

@@ -374,7 +374,7 @@ void mp_raw_code_save(mp_raw_code_t *rc, mp_print_t *print) {
 // here we define mp_raw_code_save_file depending on the port
 // TODO abstract this away properly
 
-#if defined(__i386__) || defined(__x86_64__) || (defined(__arm__) && (defined(__unix__)))
+#if defined(__i386__) || defined(__x86_64__) || defined(__unix__)
 
 #include <unistd.h>
 #include <sys/stat.h>