Bug#905346: sbuild should work around missing cross compilation dependencies

[Helmut Grohne <helmut@subdivi.de>]

Package: sbuild
Version: 0.77.0-4
Severity: wishlist

During the DebConf18 cross bof it was mentioned that cross building with
sbuild is still difficult, because you need to remember so many options.
One of those options is --add-depends="libc-dev, libstdc++-dev". This
option is necessary, because these dependencies are presently missing
from the relevant crossbuild-essential-$arch packages. Adding them there
would trip them up in britney and prevent testing migration. The problem
is reported as #815172. It has persisted for quite a while now and a
solution will take time (likely it will not be fixed in buster).
pbuilder already works around this limitation. I propose that sbuild
does the same and adds these dependencies to the default that now
replaced $crossbuild_core_depends.

Helmut