elf: Fix tst-nodeps2 test failure.

After 78ca44da
("elf: Relocate libc.so early during startup and dlmopen (bug 31083)")
we start seeing tst-nodeps2 failures when building the testsuite with
--enable-hard-coded-path-in-tests.

When building the testsuite with --enable-hard-coded-path-in-tests
the tst-nodeps2-mod.so is not built with the required DT_RUNPATH
values and the test escapes the test framework and loads the system
libraries and aborts. The fix is to use the existing
$(link-test-modules-rpath-link) variable to set DT_RUNPATH correctly.

No regressions on x86_64.

Reviewed-by: Florian Weimer <fweimer@redhat.com>