Add one more "late metadata"/"digest" file to rlib archives

[petrochenkov]

Proposal

(This is an MCP for rust-lang/rust#138243)

The new file should contain data about:

• Native libraries bundled into the rlib, to avoid looking up native libraries during the regular "early" metadata construction, when it's unnecessary and cannot be done fully correctly (#138170)
• Object files bundled into the rlib, to support link time cfg on native libraries bundled as object files
• Rust object files in the rlib to avoid hacks like looks_like_rust_object_file during LTO

Unlike regular metadata which is constructed early, this additional digest will be constructed and added to the archive in the very end, when all other members of the rlib archive are already added.
It is also going to be read only when rustc is going to link the rlib into something, not earlier.

The data could potentially use some text format like json, but it would probably be simpler to just use the same format as the regular metadata uses.
The implementation can also mirror what is done for regular metadata, in a simplified form.

Me and @belovdv wanted to do this when implementing +bundle,+whole-archive (rust-lang/rust#113301) and packed_bundled_libs (rust-lang/rust#108081), as a more principle solution, but ended up with just some workarounds instead, and never returned to this.

Mentors or Reviewers

@petrochenkov

Process

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[rustbot]

Important

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