Auto merge of #11945 - gpoesia:master, r=SimonSapin

Add mach build-stable to build with stable rustc

<!-- Please describe your changes on the following line: -->

Added mach subcommand (build-stable) to build servo and its dependencies using a stable version of rustc.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #11806 (github issue number if applicable).

<!-- Either: -->
- [X] These changes do not require tests because they're in the build process. I've manually tested both build and build-stable several times and in different orders and it seems to work. If the mach subcommands are tested automatically somewhere, I'll be happy to add a test, but I couldn't find it.

This is an example of output I get, which is what I'd expect:

```plain
[gpoesia@gabriel-notebook servo]$ ./mach build
looking for rustc at /home/gpoesia/dev/servo/.servo/rust/2016-06-24/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/rustc
Downloading Rust compiler...
Downloading Rust compiler: 100.0%
Extracting Rust compiler...
Rust compiler ready.
Downloading Host rust library for target x86_64-unknown-linux-gnu...
Downloading Host rust library for target x86_64-unknown-linux-gnu: 100.0%
Extracting Rust stdlib for target x86_64-unknown-linux-gnu...
Rust x86_64-unknown-linux-gnu libs ready.
Build completed in 0:00:01
[gpoesia@gabriel-notebook servo]$ ./mach build
Build completed in 0:00:02
[gpoesia@gabriel-notebook servo]$ ./mach build-stable
looking for rustc at /home/gpoesia/dev/servo/.servo/rust/rustc-1.9.0-x86_64-unknown-linux-gnu/rustc/bin/rustc
Rust compiler already downloaded. Use |bootstrap-rust --force| to download again.
Rust lib for target x86_64-unknown-linux-gnu already downloaded. Use |bootstrap-rust --force| to download again.
   Compiling log v0.3.6
   Compiling tenacious v0.2.1
   Compiling traitobject v0.0.1
   Compiling azure v0.4.6 (https://github.com/servo/rust-azure#4d72934a)
   Compiling crossbeam v0.2.9
/home/gpoesia/dev/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/tenacious-0.2.1/src/lib.rs:1:1: 1:42 error: #[feature] may not be used on the stable release channel
/home/gpoesia/dev/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/tenacious-0.2.1/src/lib.rs:1 #![feature(plugin_registrar, box_syntax)]
                                                                                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/gpoesia/dev/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/tenacious-0.2.1/src/lib.rs:3:1: 3:27 error: #[feature] may not be used on the stable release channel
/home/gpoesia/dev/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/tenacious-0.2.1/src/lib.rs:3 #![feature(rustc_private)]
                                                                                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to 2 previous errors
Build failed, waiting for other jobs to finish...
error: Could not compile `tenacious`.

To learn more, run the command again with --verbose.
Build completed in 0:00:03
[gpoesia@gabriel-notebook servo]$ ./mach build-stable
looking for rustc at /home/gpoesia/dev/servo/.servo/rust/rustc-1.9.0-x86_64-unknown-linux-gnu/rustc/bin/rustc
Rust compiler already downloaded. Use |bootstrap-rust --force| to download again.
Rust lib for target x86_64-unknown-linux-gnu already downloaded. Use |bootstrap-rust --force| to download again.
   Compiling js v0.1.3 (https://github.com/servo/rust-mozjs#707bfb4f)
   Compiling serde_item v0.2.0
   Compiling encoding-index-tradchinese v1.20141219.5
   Compiling angle v0.1.0 (https://github.com/servo/angle?branch=servo#d0a2db05)
/home/gpoesia/dev/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/serde_item-0.2.0/src/lib.rs:3:43: 3:74 error: #[feature] may not be used on the stable release channel
/home/gpoesia/dev/servo/.cargo/registry/src/github.com-1ecc6299db9ec823/serde_item-0.2.0/src/lib.rs:3 #![cfg_attr(not(feature = "with-syntex"), feature(rustc_private, plugin))]
                                                                                                                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error
Build failed, waiting for other jobs to finish...
error: Could not compile `serde_item`.

To learn more, run the command again with --verbose.
Build completed in 0:00:37
[gpoesia@gabriel-notebook servo]$
```

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

Github issue: #11806
Building with current stable rust (1.9.0) still fails because of
feature pragmas in some dependencies (e.g. serde_item).

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/11945)
<!-- Reviewable:end -->

0 files changed, 0 insertions, 0 deletions