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().
Resolves: rhbz#2216906