Auto merge of #37936 - tedsta:fuchsia_std_process, r=alexcrichton

Auto merge of #37936 - tedsta:fuchsia_std_process, r=alexcrichton

Fuchsia support for std::process via liblaunchpad.

Now we can launch processes on Fuchsia via the Rust standard library! ... Mostly.

Right now, ~5% of the time, reading the stdout/stderr off the pipes will fail. Some Magenta kernel people think it's probably a bug in Magenta's pipes. I wrote a unit test that demonstrates the issue in C, which I was told will expedite a fix. https://fuchsia-review.googlesource.com/#/c/15628/

Hopefully this can get merged once the issue is fixed :)

@raphlinus