cc-wrapper: fix guessing mainProgram

ccName points to the pname of the unwrapped cc minus the targetPrefix,
thus is always incorrect for cross compilers whose wrappers are prefixed
with the target platform name.

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

@@ -759,6 +759,6 @@ stdenvNoCC.mkDerivation {
     (optionalAttrs (cc_ ? meta) (removeAttrs cc.meta ["priority"])) //
     { description = attrByPath ["meta" "description"] "System C compiler" cc_ + " (wrapper script)";
       priority = 10;
-      mainProgram = if name != "" then name else ccName;
+      mainProgram = if name != "" then name else "${targetPrefix}${ccName}";
   };
 }