Fix 32-bit build after r367229.

The use of atomic_sub_64() in zfs_zstd.c was breaking the 32-bit build on
platforms without native 64-bit atomics due to atomic_sub_64() not being
available, and no fallback being provided in _STANDALONE.

Provide a standalone stub to match atomic_add_64() using simple math.

While this is not actually atomic, it does not matter in libsa context,
since it always runs single-threaded and does not run under a scheduler.

Reviewed by: mjg (in email)

sys/contrib/openzfs/include/os/freebsd/spl/sys/atomic.h

@@ -188,6 +188,8 @@ atomic_cas_ptr(volatile void *target, void *cmp,  void *newval)
  */
 #undef atomic_add_64
 #define	atomic_add_64(ptr, val) *(ptr) += val
+#undef atomic_sub_64
+#define	atomic_sub_64(ptr, val) *(ptr) -= val
 #endif /* !_STANDALONE */
 
 #endif	/* !_OPENSOLARIS_SYS_ATOMIC_H_ */