rustc should print an unambiguous error message when building no_std binaries with panic=unwind

[Trolldemorted]

As we have found out in #45492, rustc cannot (yet) build a no_std binary when compiling with panic=unwind.

Instead of aborting and printing an error message, rustc builds objects that have external dependencies, in msvc's case __CxxFrameHandler3.

[Mark-Simulacrum]

Today we say "error: #[panic_handler] function required, but not found" which seems basically perfect since that'll lead you to https://doc.rust-lang.org/nomicon/panic-handler.html which'll hopefully make it mostly clear what needs to happen.