Enable unreachable_pub lint in core

[Urgau]

This PR enables the unreachable_pub as warn in core, rtstartup and panic_unwind.

The motivation is similar to the compiler MCP: Enable deny(unreachable_pub) on rustc_* crates :

"Where is this thing used?" is a question I ask all the time when reading unfamiliar code. Because of this, I generally find it annoying when things are marked with a more permissive visibility than necessary. "This thing marked pub, which other crates is it used in? Oh, it's not used in any other crates."

Another motivation is to help to lint by utilizing it in-tree and seeing it's limitation in more complex scenarios.

The diff was mostly generated with ./x.py fix --stage 1 library/core/ -- --broken-code, as well as manual edits for code in macros, generated code and other targets.

r? libs