make-bootstrap-tools-cross.nix: Fix gcc build-time options for armv5tel

I broke this in the cleanups I did in 171c7f0, the gcc inside the bootstrap
tarball is not getting built with the correct --with-fpu, --with-float
etc. options.

pkgs/stdenv/linux/make-bootstrap-tools-cross.nix

···
       libc = "glibc";
       platform = pkgsNoParams.platforms.sheevaplug;
       openssl.system = "linux-generic32";
+      inherit (platform) gcc;
     };
   };