[libunwind] [libunwind][cmake] Compile _Unwind* routines with -fexceptions (PR #121819)

[via cfe-commits]

Andarwinux wrote:

> I wonder if it would be less bug-prone in the long term to just build the whole library with `-fexceptions` instead of trying to apply `-fexceptions` to _just_ the right files at this point in time. Feels like a brittle optimization for not much gain?

This will break MinGW LTO libunwind.

https://github.com/llvm/llvm-project/pull/121819