`--dynlibdir` indicates the directory in which dynamic libraries
are installed. By default this setting is equal to:

`$libdir/$abi`

The static libraries will still end up in:

`$libdir/$libsubdir`

With `$libsubdir/$abi` as the default directory for dynamic
libraries, dynamic libraries will by default end up in a
single shared directory (per package database). This has the
potential to reduce start-up times for dynamically linked
executable as only one RPATH per package database will be
needed.

This commit uses the functionality defined in

https://phabricator.haskell.org/D2611

to tell GHC's > 8.0.1 package database that dynamic libraries
are copied to the directories mentioned in the

`dynamic-library-dirs`

field.