uclibc: disable PIE for microblaze

PIE is not supported at the moment for uClibc-ng/microblaze.
Fixes:
http://autobuild.buildroot.net/results/a5848753eb0e2b0ddf55e226c45b85fd3a1c84f3/



Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>