nptl: Make tst-tls3mod.so explicitly lazy

Fixes the following test-time errors, that lead to FAILs, on toolchains
that set -z now out o the box, such as the one used on Gentoo Hardened:

  .../build-x86-x86_64-pc-linux-gnu-nptl $ grep '' nptl/tst-tls3*.out
  nptl/tst-tls3.out:dlopen failed
  nptl/tst-tls3-malloc.out:dlopen failed

Reviewed-by: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Andreas K. Hüttel <dilfridge@gentoo.org>