Regression: gcc-avr C++ non-functional in 3.21

avr-g++ no longer works correctly in 3.21:

/tmp % cat a.cpp
void f() { static_assert(true); }
/tmp % podman run -it -v /tmp:/tmp alpine:3.20
/ # apk add gcc-avr
[...]
/ # avr-g++ -c -std=c++20 /tmp/a.cpp 
/ # exit
/tmp % podman run -it -v /tmp:/tmp alpine:3.21
/ # apk add gcc-avr
[...]
/ # avr-g++ -c -std=c++20 /tmp/a.cpp 
cc1plus: warning: '-Werror=' argument '-std=c++20' is not valid for C++
cc1plus: warning: command-line option '-fno-rtti' is valid for D but not for C++
cc1plus: warning: '-Werror=' argument '-fno-enforce-eh-specs' is not valid for C++
/ #