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]

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[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.