* llvm: make targets a multivalued variant
* Fix the targets variant values
1. Make them lowercase and add a mapping to cmake equivalent
2. auto -> all
2. Restore composability by using a multivalued variant, so that
`targets=all` and `targets=x86` is combined to `targets=all,x86`
which is then transformed into LLVM_TARGETS_TO_BUILD=all.
* use targets=x86 in iwyu
* Default to nvptx/amdgpu/host arch targets
* default to none
* Update var/spack/repos/builtin/packages/zig/package.py
071778b919