[libunwind] [libunwind] Compile the asm as well as the C++ source (PR #86351)

[Alexander Richardson via cfe-commits]

arichardson wrote:

It might make sense to restore the `message(FATAL_ERROR)` since that is any easy mistake to make considering it used to be the default way of building libunwind.

https://github.com/llvm/llvm-project/pull/86351