[libunwind] [libunwind] Detect cycles of length 1 (PR #103476)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> Are you able to insert a `.cfi_undefined <return_register>` in the generated assembly? That would tell libunwind to stop unwinding and is cleaner than this workaround IMO.
> 
> I just noticed a similar problem in FreeBSD's startup files but I plan to fix those in FreeBSD instead of working around them here.

Thanks for weighing in. I'll close this PR. I agree that sending a patch to musl is worth trying:

> (And, IIUC, musl people generally don't want programs to be able to unwind their own stack for debugging purposes. If I send a fix to musl, I imagine the answer may be "gdb already works, libunwind already doesn't work, and that's exactly how we want it", but may be worth a try.)

https://github.com/llvm/llvm-project/pull/103476