community/gcc-cross-embedded: Fix linking with C++

Fully standard compliant support for C++ global deconstructors require the use of _cxa_atexit rather than atexit for registration, which is not available on bare metal embedded systems. So let's disable this to prevent linking issues.

Note that global deconstructors make little sense on microcontrollers that run a firmware rather than user space programs - so where no sensible "program exit" event exits - make little sense anyway.