mandelbrot: fix problem with junk at bottom
mandelbrot: fix problem with junk at bottom

I changed the behavior of crt0 so it halts when any thread
returns. Need to synchronize to ensure all threads are complete
at the end of main so the first thread doesn't kill the others.

[ci skip]

jbush001/NyuziProcessor,jbush001/NyuziProcessor,FulcronZ/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor,FulcronZ/NyuziProcessor,FulcronZ/NyuziProcessor,jbush001/NyuziProcessor,FulcronZ/NyuziProcessor,jbush001/NyuziProcessor