gcc*: always force --program-prefix and pass --target

Don't rely on gcc detecting from the passed platforms which prefix to
use, but always specify the prefix nixpkgs expects (or doesn't). This
allows us to work around problems where the configure script would add
prefix where nixpkgs doesn't expect one (if `--target` was specified,
but the same as `--host`) or doesn't add one if nixpkgs expects one (if
`--target` and `--host` are the same, but we are actually cross
compiling, but the relevant parts of the platform are not encoded into
the platform config.

See also ca9be0511b.

pkgs/development/compilers/gcc/7/default.nix

@@ -197,8 +197,7 @@ stdenv.mkDerivation ({
 
   dontDisableStatic = true;
 
-  # TODO(@Ericson2314): Always pass "--target" and always prefix.
-  configurePlatforms = [ "build" "host" ] ++ lib.optional (targetPlatform != hostPlatform) "target";
+  configurePlatforms = [ "build" "host" "target" ];
 
   configureFlags = import ../common/configure-flags.nix {
     inherit