Assorted minor improvements:
- Add a build conflict for commonmark-cmark-* when DOCS are enabled. This prevents a failure later on in the build. 
- Add a new option BE_AMDGPU which can be used to enable the AMDGPU backed used by mesa when BE_NATIVE or BE_FREEBSD is set. Enable this option by default to limit later surprises. 
- Use LLVM_ENABLE_Z3_SOLVER instead of the now removed CLANG_ANALYZER_ENABLE_Z3_SOLVER to disable Z3 discovery and linkage.
PR: 239636 , 230789