Add test case for [BZ #19329]

Test concurrent dlopen and pthread_create when the loaded
modules have TLS.  This triggers dl-tls assertion failures
more reliably than the tst-stack4 test.

The dlopened module has 100 DT_NEEDED dependencies, the
number of concurrent threads created during dlopen depends
on filesystem speed and hardware. At most 3 threads exist
at a time to limit resource usage.

Doing the test in a fork loop can make it more reliable.

---
v4:
- rebased, updated copyright year.
- moved to tests-internal because of <atomic.h>

v3:
- use the new test support code.
- better Makefile usage so modules are cleaned properly.

v2:
- undef NDEBUG.
- join nop threads so at most 3 threads exist at a time.
- remove stack size setting (resource usage is no longer a concern).
- stop creating threads after dlopen observably finished.
- print the number of threads created for debugging.

2016-12-13  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	* nptl/Makefile (tests): Add tst-tls7.
	(modules-names): Add tst-tls7mod, tst-tls7mod-dep.
	* nptl/tst-tls7.c: New file.
	* nptl/tst-tls7mod-dep.c: New file.
	* nptl/tst-tls7mod.c: New file.