Fix @threadcall argument passing.

[maleadt]

Fixes #17819. Also document not being allowed to call back into Julia.

[yuyichao]

The requirement is basically #17573, do we have that documented already?

[kshyatt]

I've restarted the OSX build which seems to have failed for other reasons. If that passes, is this PR good to go?

[tkelman]

when you restart a travis build, please back up the log to a gist first. travis overwrites logs when you restart.

[StefanKarpinski]

What even is the point of these OS X builds? Can we merge this?

[StefanKarpinski]

@yuyichao: even though that's the underlying cause of this restriction, it doesn't hurt to mention in the docstring for @threadcall too.

[kshyatt]

when you restart a travis build, please back up the log to a gist first. travis overwrites logs when you restart.

Apologies, will do next time.

[maleadt]

Yeah, this should be good to go. I had restarted the OS X bot already (to no avail), but forgot to mention it.

[yuyichao]

@StefanKarpinski I'm only asking do we have that documented already?

[tkelman]

when something persistently fails, look more into the reasons next time

[maleadt]

I did, and even though I should have spotted the ccall failure, the logs always ended in:

ErrorException("could not load library "libopenblas"
dlopen(libopenblas.dylib, 1): image not found")
    From worker 8:       * examples              in  33.75 seconds, maxrss  474.41 MB
The job exceeded the maximum time limit for jobs, and has been terminated.

So I assumed the failure to be unrelated (as it often happens)... Will be more careful next time.