Swap multiarch-support for a defensive dependency on libc6

[bossmc]

As of Ubuntu Focal Fossa (20.04) the transitional multiarch-support package (introduced in Natty Narwhal) has finally been removed. This MR removes the explicit dependency on that package which unblocks adding a Focal Fossa build of the library.

The details of the change required quite a bit of research so I've written up my findings and logic here:

Back in 2006 glibc added a model for separating 32bit/64bit libraries between separate library paths (/usr/lib/x86_64-linux-gnu vs /usr/lib/i386-linux-gnu). Debian (and thus Ubuntu) adopted that policy into their package manager around the time of the Natty Narwhal release, but any packages that used the new paths needed a version of ld-linux.so that knew to look in the architecture-specific paths for dynamic libraries. To make this easier the multiarch-support package was added, which is effectively an alias for a dependency on libc6 >= 2.3.6-2 (since 2.3.6-2 is the version that enabled the multiarch folder searching), the idea being that packages that use the new paths (which in time was expected to be all packages that install libraries) would Pre-Depend on multiarch-support thereby ensuring that their new libraries would be findable after installation.

Ubuntu releases since Edgy Eft have all provided a new enough libc6 by standard, so the multiarch-support package has been redundant for new installs (since 9 years before it was even created!), but it's been kept around to handle installs that have been upgraded from even older releases to make sure libc6 is sufficiently upgraded. As of Focal Fossa this defensive package has been removed (I assume since there's no upgrade path from pre-Dapper Drake that won't sort out the libc6 versioning at some point).

I considered just removing the dependency outright, but - since in theory it's the "right thing to do" on Xenial Xerus/Bionic Beaver, I instead replaced it with the dependency on the appropriate libc6 package, which I claim will be a no-op on every install of this package ever 😀.