stdlib: Allow concurrent quick_exit (BZ 31997)

As for exit, also allows concurrent quick_exit to avoid race
conditions when it is called concurrently.  Since it uses the same
internal function as exit, the __exit_lock lock is moved to
__run_exit_handlers.  It also solved a potential concurrent when
calling exit and quick_exit concurrently.

The test case 'expected' is expanded to a value larger than the
minimum required by C/POSIX (32 entries) so at_quick_exit() will
require libc to allocate a new block.  This makes the test mre likely to
trigger concurrent issues (through free() at __run_exit_handlers)
if quick_exit() interacts with the at_quick_exit list concurrently.

This is also the latest interpretation of the Austin Ticket [1].

Checked on x86_64-linux-gnu.

[1] https://austingroupbugs.net/view.php?id=1845


Reviewed-by: Carlos O'Donell <carlos@redhat.com>