cmd: enable stacksplit
cmd: enable stacksplit

Enable stacksplit on sparc64 as it seems to work for relatively small
stacks reliably; larger stacks still need more work, but there's little
point in leaving this commented out since it's actively being worked on.

4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go