[compiler-rt] [compiler-rt] Fix CMake configure on Windows (PR #90843)

[David Spickett via llvm-commits]

DavidSpickett wrote:

> This problem was not caught on Windows buildbots beacuase it appeared when compiler-rt was included via LLVM_ENABLE_PROJECTS while buildbots include compiler-rt project using LLVM_ENABLE_RUNTIMES flag.

Surprising, but the bot is green so I assume `LLVM_ENABLE_RUNTIMES` doesn't need any fixes?

(`LLVM_ENABLE_RUNTIMES` is the recommended way going forward, but `LLVM_ENABLE_PROJECTS` is still valid and worth fixing for sure)

https://github.com/llvm/llvm-project/pull/90843