use default binutils for microblaze

Upstream binutils 2.24 works fine for microblaze, no need for
Xilinx Git.

[Peter: disable older versions as suggested by Gustavo]
Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>