The toolchain dependencies on gcc and gfortran cannot be satisfied
during cross compilation. The gcc one is always satisfied natively,
because gcc is build essential and the version lacks the epoch.
Accounting for the epoch, it would be satisfied in bookworm. Hence I
suggest dropping it. For the gfortran dependency, we need to add a
"-for-host" suffix. At that point, we must prefix the compiler with the
host architecture even for native builds. dpkg's buildtools.mk may be
used to initialize suitable environment variables. Consider applying the
attached patch for forky.

Helmut