Fix "elf: Add tst-ldconfig-ld_so_conf-update test" on 32bit.

This new test was introduced with recent commit
591236f1.
If run on 32bit, it fails while renaming tst-ldconfig-ld-mod.so as there is no
/usr/lib64 directory. This patch is constructing the file name with help of
support_libdir_prefix.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>