Since about forever, build-essential depends on libc6-dev-sparc64
[sparc]. No other architecture does it like that and this dependency
becomes difficult during architecture cross bootstrapping. I talked to
John Paul Adrian Glaubitz who seems to be about the only one still
supporting sparc/sparc64 and we agreed to drop this dependency:
https://github.com/sparclinux/issues/issues/57

This will cause an unknown amount of packages to FTBFS on sparc, but
sparc isn't even a ports architecture anymore. It is not autobuilt at
all. We will not be able to tell which. If we run into any, adding the
dependency will be an easy thing to do. On the flip side, we expect that
most packages don't need libc6-dev-sparc64. In particular, we expect
that none of the packages relevant to bootstrapping need it. So let's
just remove this dependency and defer handling the fallout.

Patch attached.

Helmut