Fix cross compilation (nm command)

Commit c261d774fb9093d00e0938a19f502fb220f62718 added one more binutils
tool: nm also needs a cross prefix.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1411070108-8954-1-git-send-email-sw@weilnetz.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>