Enables option split-debuginfo on all platforms

[alex-semenyuk]

Closes: #96643

[rust-highfive]

r? @Mark-Simulacrum

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[bors]

☔ The latest upstream changes (presumably #96695) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #96804) made this pull request unmergeable. Please resolve the merge conflicts.

[Mark-Simulacrum]

I think this will fail on platforms where the flag is already stable, or at least give warnings -- I'm going to go ahead and close in favor of #96758 which should have fixed the known bug for BSD platforms at least. I think it may be worth considering in the future that platform-specific stability is a bad idea here, and instead we should (for example) have stable access through -C... but keep -Z... as unstable across all platforms until all/vast majority of platforms stabilize.

(cc @davidtwco)