-Zdebug-macros should either be the default or continue to be available on release-channel compiler

[hsivonen]

Firefox is built with release-channel Rust, and -Zdebug-macros is needed to get useful debuggability, crash/panic reporting and profiling attributability for Rust code in Firefox.

According to build-time output, the option is going to go away from the release-channel compiler. It seems that the option isn't on track to becoming the default, either.

If it's not made the default, please continue keeping the option (under some name) available to release-channel Rust.

[LucasFA]

As I understand it, #100758 closes this. @rustbot label +F-collapse_debuginfo +A-debuginfo.