Auto merge of #39252 - alexcrichton:less-exports, r=nrc

Hide a few more standard library symbols

These commits touch up some of the symbol visibility rules for some crates related to the standard library, notably:

* Symbols that are `pub extern` and `#[no_mangle]` which are internal-to-rust ABI things are no longer at the `C` export level, but the `Rust` export level. This includes allocators, panic runtimes, and compiler builtins.
* The libbacktrace library is now compiled with `-fvisibility=hidden` to ensure that we don't export those symbols.