arm-none-eabi toolchain and compiling C++ stuff

[Rutherther <rutherther@protonmail.com>]

Hello Attila,

lately I've been trying to make arm-none-eabi toolchain with gcc 12.3,
since the older ones cannot be used with QMK. I also stumbled accross
this issue. The issue is actually caused by wrong order of paths in
"CROSS_CPLUS_INCLUDE_PATH". The C include path has to come after the C++ path.
So as a workaround, remove the /include from beginning of this, and
append it instead as last.

Apart from that you will probably have an issue with linking against the
libstdc++, since it's inside of "lib" folder instead or "arm-none-eabi/lib"
that is in CROSS_LIBRARY_PATH. As a workaround, just add
"$GUIX_ENVIRONMENT/lib" to "CROSS_LIBRARY_PATH".

I will try to submit the 12.3 patch soon. In the mean time I have it
inside of my own channel https://github.com/Rutherther/guix-exprs/blob/main/ruther/packages/embedded.scm

Redards,
Rutherther