[llvm] [llvm-ar][Object] Use K_GNU instead of K_COFF for archives with no symtab. (PR #82898)

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo approved this pull request.

The changes look reasonable to me, so LGTM. But the PR subject+description (which end up as commit message for the new commit) makes this a bit hard to understand - I'd prefer to rephrase it as a copy of the original commit message (which is the main effect of the commit once it lands on main), while the incremental changes from the previous PR just is a minor detail in the big picture. I.e. "Reapply [llvm-ar] Use COFF archive format ...", same commit message as the previous one, but with the story of original PR, revert (and reason for it) and reapplication (with what is changed to remedy the issue) as an extra final paragraph in that commit message.

https://github.com/llvm/llvm-project/pull/82898