Add --print-sysroot commandline option to rustc

[michaelwoerister]

This is in support of the coming "rust-mode" debugger startup scripts.
The implementation given here will print the sysroot if no input filename is given to rustc. Otherwise the option is silently ignored. Let me know if you'd like to have a different behavior.

cc @brson @alexcrichton

[brson]

@alexcrichton I wonder if it might be prudent to hide this behind a -Z flag. I'm getting a little concerned about command line flag proliferation...

[alexcrichton]

Hm we do seem to grow quite the number of flags over time. I suppose I don't quite know what the -Z flag is used for, but I'm also fine throwing it behind there. We do have some other related flags for printing like:

    --print-crate-name  Output the crate name and exit
    --print-file-name   Output the file(s) that would be written if
                        compilation continued and exit

so there is at least some precedent for the top level (but not necessarily good precedent)

[michaelwoerister]

I'm OK with either variant and the -Z flag option being the more cautious one, maybe we should just do that.

[brson]

Yeah, the -Z option's purpose is somewhat unclear, and I feel like it's changed over time. Let's put it there for now to be conservative. In the near future we're probably going to take a hard look at the compiler interface itself from a backwards-compatibility standpoint. This is easy enough to revisit in the future if more use cases come up.

[michaelwoerister]

I'll close this PR for now. I have a pretty much complete implementation of the LLDB wrapper script and it's being installed to the right places here: https://github.com/michaelwoerister/rust/tree/lldb-scripts
It just needs some testing. I'll add the -Z print-sysroot option together with the other script-related commits in a new PR next week.