libnss: workaround for microblaze ld bug

Add a workaround for a microblaze specific ld bug to avoid
segfault.

Fixes:
  http://autobuild.buildroot.net/results/638a65453879777a0d5bdb29231034cd261b41c0



[Peter: rename to LIBNSS_DROP_GC_SECTIONS for clarity]
Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>