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

[Alexander Richardson via cfe-commits]

arichardson 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.

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