Auto merge of #148339 - folkertdev:naked-macos-private-extern, r=Amanieu

naked functions: emit `.private_extern` on macos

fixes rust-lang/rust#148307

Emit `.private_extern` on macos when the naked function uses `Linkage::Internal`. Failing to do so can cause issues with LTO.

The documentation on this directive is kind of sparse, but I believe this is at least not incorrect, and does fix the issue.

r? @Amanieu
cc @bjorn3

Author: rust-bors[bot]