Compiler memory barriers do not prevent the CPU from executing the code
out of order. Switch to C11 atomics. This also lets us get rid of the
mutex; instead, loop until the compare_exchange succeeds.
While here, change the return value of at_quick_exit() on failure to
the more traditional -1, matching atexit().
Sponsored by: Klara, Inc.
Reviewed by: Olivier Certner, kevans, kib
Differential Revision: https://reviews.freebsd.org/D41936
(cherry picked from commit 1dc3abb052430279e47c8922d22b30922adcf0f6)
libc: Add a rudimentary test for quick_exit(3).
Sponsored by: Klara, Inc.
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D41937
(cherry picked from commit c7dd4601aeebbc1bbe131cbe6747476c124b47fe)
