io: disable i686 build

io uses SIMD instructions even on i686, causing the build to fail:

> /nix/store/[...]-gcc-5.4.0/lib/gcc/i686-pc-linux-gnu/5.4.0/include/xmmintrin.h:181:1:
error: inlining failed in call to always_inline '_mm_add_ps': target
specific option mismatch
 _mm_add_ps (__m128 __A, __m128 __B)

(from https://hydra.nixos.org/build/37879114/log/raw)

The simplest solution is to disable the build on this platform.

pkgs/development/interpreters/io/default.nix

@@ -47,6 +47,6 @@ stdenv.mkDerivation {
       z77z
       vrthra
     ];
-    platforms = platforms.linux;
+    platforms = [ "x86_64-linux" ];
   };
 }