Update Ubuntu version for {x86_64, i686}-unknown-linux-gnu docker images

[Disasm]

This should solve svd2rust CI build issues.

Also Rust uses Ubuntu 16.04 in x86_64-gnu-distcheck Docker image.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @jamesmunns (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[Disasm]

(Yes, I know about this, but It's time to move forward)

[Dylan-DPC-zz]

bors: try

[Disasm]

New problem here: https://github.com/rust-embedded/cross/blob/42fd2420fb7f568e10cd534725245df8ecae8904/ci/script.sh#L33-L43

error: failed to run custom build command for `compiler_builtins v0.1.15 (/project)`

Caused by:
  process didn't exit successfully: `/target/debug/build/compiler_builtins-c292f450b529b894/build-script-build` (exit code: 101)
--- stdout
cargo:rerun-if-changed=build.rs
cargo:compiler-rt=/project/compiler-rt

--- stderr
thread 'main' panicked at 'RUST_COMPILER_RT_ROOT is not set', build.rs:420:21
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

Introduced 2 days ago: rust-lang/compiler-builtins#294
I have no idea how to fix this.

[bors]

try

Build failed

• continuous-integration/travis-ci/push

[japaric]

Also Rust uses Ubuntu 16.04 in x86_64-gnu-distcheck Docker image.

This is not the docker image used to build the rust-std components. It's only used for testing.

The image used to build the rust-std components is this centos 5 image.

This should solve svd2rust CI build issues.

rust-embedded/svd2rust#297 should fix those

[Disasm]

Ok, now at least we have an alternative. Thanks!