Build "julia shipped libraries" package

[staticfloat]

With each Julia release, we should generate a BinaryProvider-style tarball of shared libraries that typically ship with Julia. Things like libgfortran.so.4, libstdc++.so.6, etc... and enable automatic downloading of it to the julia private shlib location. These will then override the typical system libraries, and since they will be newer (and backwards-compatible) they should enable from-source builders of Julia to use the precompiled binaries that we ship through BinaryBuilder.

We will have to update this package of libraries every time the compilers we're using change something (a la libgfortran.so.3 -> libgfortran.so.4 in GCC 7) but hopefully we can keep those problems limited to a particular Julia major release.