cc-wrapper: allow forcing targetPrefix or passing a custom one

This can be useful for "toolchain-style" packages which don't let us set
a custom prefix.

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

@@ -17,6 +17,12 @@
 , isGNU ? false, isClang ? cc.isClang or false, gnugrep ? null
 , buildPackages ? {}
 , libcxx ? null
+  # Prefix for binaries. Customarily ends with a dash separator.
+  #
+  # TODO(@Ericson2314) Make unconditional, or optional but always true by
+  # default.
+, targetPrefix ? with stdenvNoCC;
+    lib.optionalString (targetPlatform != hostPlatform) (targetPlatform.config + "-")
 }:
 
 with lib;
@@ -31,13 +37,6 @@ let
   stdenv = stdenvNoCC;
   inherit (stdenv) hostPlatform targetPlatform;
 
-  # Prefix for binaries. Customarily ends with a dash separator.
-  #
-  # TODO(@Ericson2314) Make unconditional, or optional but always true by
-  # default.
-  targetPrefix = lib.optionalString (targetPlatform != hostPlatform)
-                                           (targetPlatform.config + "-");
-
   ccVersion = lib.getVersion cc;
   ccName = lib.removePrefix targetPrefix (lib.getName cc);