[libunwind] [llvm] [runtimes] remove workaround for old CMake when setting `--unwindlib=none` (PR #93429)

[Louis Dionne via cfe-commits]

ldionne wrote:

I looked into it and I don't quite understand. We should definitely be building with `-fno-exceptions` in that configuration (and I can see that in the logs), so I don't understand why we have things like:

```
| ld.lld: error: undefined symbol: __cxa_begin_catch
| >>> referenced by cxa_handlers.cpp:63 (/home/tcwg-buildbot/worker/linaro-aarch64-libcxx-02/llvm-project/libcxx-ci/build/armv7m-picolibc-no-exceptions/../../libcxxabi/src/cxa_handlers.cpp:63)
| >>>               cxa_handlers.cpp.obj:(std::__terminate(void (*)())) in archive /home/tcwg-buildbot/worker/linaro-aarch64-libcxx-02/llvm-project/libcxx-ci/build/armv7m-picolibc-no-exceptions/lib/libc++abi.a
```

There should not be references to these functions since we're building without exceptions.

Pinging the bot owner @mplatings 

https://github.com/llvm/llvm-project/pull/93429