[compiler-rt] [compiler-rt][ARM] Make interworking test compatible with BTI (PR #226154)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

LGTM on my side. This test code will now run on Thumb v4t and a Thumb v8.1-m with branch protection enabled. Please leave some time before merging for other reviewers to comment.

A reference: https://developer.arm.com/community/arm-community-blogs/b/architectures-and-processors-blog/posts/armv8-1-m-pointer-authentication-and-branch-target-identification-extension 

> The “BX LR” and “BFX LR” are not BTI setting instructions because these are frequently used for function returns, and the pointer can be protected by PAC authentication.

https://github.com/llvm/llvm-project/pull/226154