In the absence of cross-translatable toolchain dependencies (#666743), a
workaround can be expanding Build-Depends. This is already used by e.g.
gcc-N and linux. If you apply this workaround to g++-multilib, it
becomes:

    g++-multilib [mipsel] <!cross>, g++-multilib-mipsel-linux-gnu [mipsel] <cross>

Unfortuntely, g++-multilib-mipsel-linux-gnu is not Multi-Arch tagged, so
a mipsel package is found, but there is none. The dependency is
unsatisfiable. Since the package name contains an architecture, it
really should be marked Multi-Arch: foreign.

mipsel is only an example here. It really works the same way for any
other architecture.

Please consider applying the attached patch.

Helmut