[RFC] [avr] Toolchain Integration for Testsuite Execution (avr cc0 to mode_cc0 conversion)

[abebeos]

Essence:

I need a confirmation that the testsuite setup as presented in:

https://github.com/abebeos/avr-gnu

works fine.

The problem with the avr target is that the testsuite cannot be run easily,
mainly because of the need for a special simulated-target setup, which does
not work for avr as documented. This led developers to a dead-end with
their non-cc0-avr-backends (the non-cc0 backend is needed thus avr is not
dropped from gcc11).

I integrated a toolchain/testsetup to be able to run the gcc testsuite
against a simulated avr target.

I then used this toolchain to test 2 different existent
non-cc0-avr-backends (from pipcet and saaadhu, both github).

The result is that saaadhu's backend seems to be working 100%. It has
identical testsuite results with the existing (but deprecated) cc0-backend,
which means that it can be used "as-is" for inclusion in gcc11.

Please note that I did this work in context of a bounty @ bountysouce, more
information within the issue:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92729#c35