Default macOS targets to `unpacked` debuginfo

Default macOS targets to `unpacked` debuginfo

This commit continues the work from #9112 to enable `unpacked` split
debuginfo on macOS targets by default. This has been discussed on [internals]
for awhile now and no breakage has emerged while significant speedups
have. This is expected to be a compile-time and `target`-directory size
win for almost all macOS Rust projects.

While breakage is possible it's possible to mitigate this with
project-local or global cargo configuration of the `dev` and `test` profiles.

[internals]: https://internals.rust-lang.org/t/help-test-faster-incremental-debug-macos-builds-on-nightly/14016/9