* elf/dl-close.c (_dl_close): When closing an object using TLS
either decrement dl_tls_max_dtv_idx or set dl_tls_dtv_gaps to
true. Increment dl_tls_generation only if we closed any TLS-using
object.
* elf/tst-tls6.c: New file.
* elf/Makefile: Add rules to build and run tst-tls6.
