Change llvm install prefix

This changes the structure of the llvm package to install files
under the `/usr/%{_lib}/llvm%{maj_ver}` prefix and add symlinks
from the system install prefix. This means that the non-compat
llvm package essentially becomes a superset of the compat llvm package.

Some notes:
 * The file lists are generated using a set of `%expand_*` macros.
   These will generate the path in the llvm prefix, plus the
   symlink from the system prefix for non-compat packages. We also
   always generate a version-suffixed symlink for binaries.
 * I've retained the old behavior regarding man pages, which are
   shipped under the system prefix, not the llvm prefix. The compat
   package only ships the man pages for the version-suffixed binaries.
 * For llvm-config, I've removed the weird `llvm-config-%{_isa_bits}`
   binaries used for multilib handling. Instead, make system llvm-config
   (via alternatives) point to the llvm-config in the llvm install
   prefix. It will be different for 64-bit and 32-bit due to the
   different libdir, so this is still safe for multilib.
 * The clang resource directory and libcxx continue to be installed in
   the system prefix, not the llvm prefix.
 * To simplify handling, I've moved the cmake utilities from
   share/llvm/cmake to share/llvm-cmake. These are for internal use
   only, so the rename should be fine.

This change is part of https://fedoraproject.org/wiki/Changes/LLVM-20.

Author: nikic