Results in "Unknown tag: %elif 0" on i686. Apparently this is not
supported on RHEL 8.
Just drop the branch as it's not relevant anyway.
Resolves: RHEL-9028
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#2223978
