Improve clang assembler error messages on s390x (rhbz#2216906)
    
    Backport a patch from LLVM 17 that improves error messages from the
    Clang assembler on s390x.
    
    Also, disable LLVM_UNREACHABLE_OPTIMIZE in order to improve error
    messages when llvm_unreachable() is called. There are many cases where
    llvm_unreachable() is used as a error reporting function instead of the
    recommended replacement for assert().
    
    Fixes rhbz#2216906.