Expand the LLVM coverage of `--print target-cpus`

We've been relying on a custom patch to add `MCSubtargetInfo::getCPUTable`
for `rustc --print target-cpus`, and just printing that it's not supported
on external LLVM builds. LLVM `main` now has `getAllProcessorDescriptions`
that can replace ours, so now we try to use that. In addition, the fallback
path can at least print the native and default cpu options.

There were also some mismatches in the function signatures here between
`LLVM_RUSTLLVM` and otherwise; this is now mitigated by sharing these
functions and only using cpp to adjust the function bodies.

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