libxml2 is required for building on Windows since LLVM 14

[gmarkall]

Identified in #922.

• @apmasell Suggested that it may be required for emitted remarks: Fix AzureCI not using llvm14 on windows #922 (comment)
• On Linux and macOS, we explicitly disable it: https://github.com/numba/llvmlite/blob/main/conda-recipes/llvmdev/build_llvm14.sh#L31

We should determine whether it's really needed and disable it if not.

[gmarkall]

I also found I needed to install libz on Windows since the update to 14 - not sure if this is a new surprise dependency too.

[sklam]

Revisiting this for llvmlite0.40.1 release. On the buildfarm, the base conda environment has an effect to the build---if it has libxml2, it gets picked up. The randomness of libxml2 requirement is coming from which windows machine got the job for the llvmdev build. One out of 3 machines has libxml2 in the base env.