[mono] Enable debugging for netcore configurations by default.

[steveisok]

What this means is:

1. --debug is no longer needed since the default is TRUE
2. --debug=ignore is a new switch to go back to the old mono default

Fixes #31662

[jaykrell]

I suggest -nodebug.

[marek-safar]

Is this really the approach we want to take? I don't see an easy way for every release build to disable this behaviour.

We could instead recognize existing COMPlus_EnableDiagnostics and convert it to full MonoVM debug mode

[lewurm]

Is this really the approach we want to take? I don't see an easy way for every release build to disable this behaviour.

For Release the tooling can pass

1. --debug=ignore, or
2. Don't care, because the performance implications aren't that bad. Having useful stack traces is imho the better default behavior anyway.

[akoeplinger]

Additionally both XI and VSMac already unconditionally pass --debug:

• https://github.com/xamarin/xamarin-macios/blob/5c4b339ed320c474f8a88a3be4435274636ad38d/tools/mtouch/mtouch.cs#L390
• https://github.com/xamarin/vsmac/blob/68e5a9cb133ee5aff44697a3c678e5ad82f0548e/main/build/MacOSX/monostub.mm#L338

[steveisok]

@marek-safar What do we want to do? I can revert / do what you suggested. However, if there are no/limited performance implications, then my vote is to keep the PR as is.

[marek-safar]

@vargaz confirmed the simple version of --debug is not that expensive so let's enable it by default.