Provide metapackage for linking against C++ standard library

As noted in #20010 (closed), it is currently extraordinarily difficult to reliably link against C++ in a portable manner. Specifically, one must encode a rather sizeable decision tree in one's cabal file, mapping common platforms to one of the two common C++ standard library implementations. Not only is this tedious, it is extremely subtle and therefore error prone. Even worse, on some platforms it isn't even possible to say which implementation a particular user will be using.

To improve this situation, this MR introduces a new "meta-package", system-cxx-std-lib, into GHC's package database. This package is configured at GHC installation-time to include the needed libraries and search paths to allow linking against C++ objects.

Fixes #20010 (closed).