Remove Linuxism from tst-tls-atexit

The tst-tls-atexit test case searches for its module in /proc/PID/maps
to verify that it is unloaded, which is a Linux-specific test.  This
patch makes the test generic by looking for the library in the link
map list in the _r_debug structure.

Verified that the test continues to succeed on x86_64.  There is a bug
in the test case where it calls dlclose once again, which is actually
incorrect but still manages to unload the DSO thanks to an existing
bug in __tls_call_dtors.  This will be fixed in a later patch which
also fixes up the __cxa_thread_atexit_impl implementation.  I have
added a FIXME comment to that call momentarily, which I will remove
when I fix the problem.

	* stdlib/tst-tls-atexit-lib.c (do_foo): Rename to reg_dtor.
	* stdlib/tst-tls-atexit.c: (is_loaded): New function.
	(spawn_thread): New function.
	(load): Rename to reg_dtor_and_close.  Move dlopen to...
	(do_test): ... here.  Use IS_LOADED to test for its
	availability.