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

[David Spickett via llvm-commits]

DavidSpickett wrote:

Linaro will look into this.

Reproducing the build is quite easy if you do want to look yourselves in the meantime:
```
<llvm project root>$ ./libcxx/utils/ci/run-buildbot armv7m-picolibc-no-exceptions
```
You'll need a recent `qemu-system-arm` installed to run the tests, I have 8.1.3 locally. Picolib is downloaded and built for you by the script.

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