hmmer: restrict platforms to x86_64

technically big-endian powerpc with VMX is also supported, but
on x86, SSE is apparently *required* which means we can't build
a pure i686 version

pkgs/applications/science/biology/hmmer/default.nix

···
     homepage = "http://hmmer.org/";
     license = licenses.gpl3;
     maintainers = [ maintainers.iimog ];
-    platforms = platforms.unix;
+    # at least SSE is *required*
+    platforms = platforms.x86_64;
   };
 }