Add --trace-compile= flag to --help-hidden output.

[NHDaly]

The --trace-compile=stderr flag was very helpful for us to understand what's going on in our system! :)

This PR just adds the flag to the list of flags in julia --help-hidden.

[NHDaly]

Was this not documented on purpose, because it's not a stable API or something? If so we could maybe just mark it EXPERIMENTAL or something?

And assuming that you don't mind this being documented, maybe I'll also add it to the docs pages?

[KristofferC]

Seems good to me to have this documented.

[KristofferC]

However, I think the docs is slightly wrong. The argument is a file name with only stderr being special cased to print to stderr (unless it has been changed since the original implementation).

[JeffBezanson]

Right, we have been very conservative about what we document. Once this goes in the help, we can't change it :|

[maleadt]

Maybe this would be a better fit for --help-hidden, seeing how not many users would ever want to use this flag? Other options there seem like they might change too.

[NHDaly]

Thanks for the advice @maleadt. Sorry it took me so long to respond to it! I've moved the help string to --help-hidden, rebased, and repushed. PTAL! :)
Thanks!