Fix universal build support for Mac and iOS

To build single-architecture binaries for 32-bit Mac:
```sh
$ LIBARCHS="i386" make
```

To build universal binaries for 32- and 64-bit Mac:
```sh
$ LIBARCHS="i386 x86_64" make
```

To build single-architecture binaries for iOS:
```sh
$ ./make.sh ios_arm64
```

To build universal binaries for iOS:
```sh
$ ./make.sh ios
```