feat: update embedded.scm based on the code submitted to Guix patches

fix: add libstdc++ CXXFLAGS, document with target subdir hack

fix: c++ include paths

The C include path has to be the latest! This is because
the cstdlib header uses #include_next <stdlib.h>, and
since the include of C headers comes first, stdlib.h is
not found!

fix: libstdc++ getentropy fix

The newlib does not expose getentropy in the final libraries,
but stil has it in the headers. That leads to configure thinking
getentropy is available and it will be used in the <random>.

feat: split arm 12.3 toolchain to non-nano and nano

fix: libstdc++ output lib path

feat: add buildable libstdc++ to arm gcc toolchain 12.3

feat: arm toolchain 12.3.rel1