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)