Fix backtraces on ARM EHABI.

[timonvo]

Before this patch, our rust_eh_personality_catch routine would cut
backtracing short at the __rust_try function, due to it not handling
the _US_FORCE_UNWIND bit properly, which is passed by libunwind
implementations on ARM EHABI.

Examples of where the _US_FORCE_UNWIND bit is passed to the PR:

• GCC's libunwind: https://github.com/gcc-mirror/gcc/blob/f1717362de1e56fe1ffab540289d7d0c6ed48b20/libgcc/unwind-arm-common.inc#L590
• LLVM's libunwind: https://github.com/llvm-mirror/libunwind/blob/61278584b5c84c422ff5da10f46c3235c54636c9/src/UnwindLevel1-gcc-ext.c#L153

[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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

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[alexcrichton]

@bors: r+ 6e41885

Thanks!

[timonvo]

Hey Alex, is this supposed to be getting merged automatically? It doesn't seem like @bors is picking it up...

[alexcrichton]

Ah yeah we just have an unfortunately large queue recently, but don't worry as this is indeed in there!