This is mostly working around a bad compiler optimization.

The Intel 80386 processor introduced some bit operations that would be
the perfect translation for atomic single-bit read-modify-and-write
operations. Alas, even the latest compilers as of today
(GCC 11, clang 13, Microsoft Visual C 19.29) would generate a loop around
LOCK CMPXCHG instead of emitting the instructions
LOCK BTS (fetch_or()), LOCK BTR (fetch_and()), LOCK BTC (fetch_xor()).

fil_space_t::clear_closing(): Clear the CLOSING flag.

fil_space_t::set_stopping_check(): Special variant of
fil_space_t::set_stopping() that will return the old value
of the STOPPING flag after atomically setting it.

fil_space_t::clear_stopping(): Use fetch_sub() to toggle
the STOPPING flag. The flag is guaranteed to be set upon
calling this function, hence we will toggle it to clear it.
On IA-32 and AMD64, this will translate into
the 80486 LOCK XADD instruction.

fil_space_t::check_pending_operations(): Replace a Boolean
variable with a goto label, to allow more compact code
generation for fil_space_t::set_stopping_check().

trx_rseg_t: Define private accessors ref_set() and ref_reset()
for setting and clearing the flags.

trx_lock_t::clear_deadlock_victim(), trx_lock_t::set_wsrep_victim():
Accessors for clearing and setting the flags.