[llvm] [libc-utils] disable target_compile_features (PR #121883)

[Louis Dionne via llvm-commits]

ldionne wrote:

> 
> We see that these variables are both empty or undefined here:
> 
> ```
> -- CMAKE_C_SIMULATE_ID is ''
> -- CMAKE_CXX_COMPILER_FRONTEND_VARIANT is ''
> ```
> 
> Are these `MSVC` checks even relevant when targeting linux?

No, but isn't that exactly what you want? Don't you want `CMAKE_CXX_COMPILER_FRONTEND_VARIANT MATCHES "MSVC"` to be false when you're on Linux?

https://github.com/llvm/llvm-project/pull/121883