Require +thumb-mode to generate thumb2 code for Android/armv7-a

[makotokato]

I am investigating rust's code generation into Gecko by https://bugzilla.mozilla.org/show_bug.cgi?id=1399337.

armv7-linux-androideabi target uses +v7,+thumb2,+vfp3,+d16,-neon as target-feature. But +thumb2 only doesn't generate thumb2 code. To generate thumb2 code, it requires +thumb-mode. So we should add it for armv7-linux-androideabi.

r? @alexcrichton

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

[alexcrichton]

@bors: r+

How odd! Looks good regardless!

[bors]

📌 Commit 07494ec has been approved by alexcrichton

[alexcrichton]

@bors: rollup

[parched]

@makotokato@alexcrichton Just noticed this, I'm not sure if it's an issue, but conventionally in rustc the arm targets that generate thumb code are named with thumb.. so maybe this target should be renamed now or this should be a separate target. Also this is a breaking change for anyone who was relying on this target to generate ARM code.