bintools-wrapper: fix unknown emulation error message

Using the + operator with throw doesn’t seems to work properly. You
need to use antiquotes here to get the targetPlatform config included.

pkgs/build-support/bintools-wrapper/default.nix

@@ -189,7 +189,7 @@ stdenv.mkDerivation {
       else if targetPlatform.isSparc then "sparc"
       else if targetPlatform.isAvr then "avr"
       else if targetPlatform.isAlpha then "alpha"
-      else throw "unknown emulation for platform: " + targetPlatform.config;
+      else throw "unknown emulation for platform: ${targetPlatform.config}";
     in targetPlatform.platform.bfdEmulation or (fmt + sep + arch);
 
   strictDeps = true;