Rollup merge of #71490 - petrochenkov:passcheck2, r=davidtwco

Cleanup and document `-C relocation-model`

As the title says, this is mostly a refactoring and documentation.

One potentially observable change here is that `-C relocation-model=default` now takes the default from the Rust target, rather than from the underlying LLVM target. In other words, `-C relocation-model=default` is now equivalent to not specifying the relocation model on command line at all.
Apparently no one used that option because it has other bugs as well, e.g. PIC `default` wasn't treated as PIC in some places.

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