Fix fallback implementation for pg_atomic_test_set_flag().

The fallback implementation of pg_atomic_test_set_flag() that uses
atomic-exchange gives pg_atomic_exchange_u32_impl() an extra
argument.  This issue has been present since the introduction of
the atomics API in commit b64d92f1a5.

Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/20231114035439.GA1809032%40nathanxps13
Backpatch-through: 12

src/include/port/atomics/generic.h

@@ -83,7 +83,7 @@ pg_atomic_init_flag_impl(volatile pg_atomic_flag *ptr)
 static inline bool
 pg_atomic_test_set_flag_impl(volatile pg_atomic_flag *ptr)
 {
-	return pg_atomic_exchange_u32_impl(ptr, &value, 1) == 0;
+	return pg_atomic_exchange_u32_impl(ptr, 1) == 0;
 }
 
 #define PG_HAVE_ATOMIC_UNLOCKED_TEST_FLAG