isa-l: (temporarily?) exclude i686-linux from platforms

It looks like s390 and i386 _should_ be supported, but are broken in
practice. The patch Alpine Linux is using looks a little bit strange.
Since isa-l is often optional, I've decided not to investigate this
further. Dropping i686-linux from platforms already takes care of the
cases where it is optional.

pkgs/by-name/is/isa-l/package.nix

···
       # <instantiation>:4:26: error: unexpected token in argument list
       #  movk x7, p4_low_b1, lsl 16
       "aarch64-darwin"
+      # https://github.com/intel/isa-l/issues/188
+      "i686-linux"
     ];
   };
 })