aften: Disables build on aarch64-linux

It looks like it would be a trivial fix with
`-Wno-shift-negative-value`, but in the end it fails with:

```
[100%] Linking C executable aften
libaften_static.a(a52enc.o): In function `aften_encode_init':
a52enc.c:(.text+0x303c): undefined reference to `apply_simd_restrictions'
collect2: error: ld returned 1 exit status
```

So it looks like it's not simply a warning issue.

pkgs/development/libraries/aften/default.nix

···
 		description = "An audio encoder which generates compressed audio streams based on ATSC A/52 specification";
 		homepage = "http://aften.sourceforge.net/";
 		license = stdenv.lib.licenses.lgpl2;
-		platforms = stdenv.lib.platforms.unix;
+		platforms = [ "i686-linux" "x86_64-linux" "x86_64-darwin" ];
 	};
 }