Log Message:
lang/gcc9: split PLIST into platform-specific files

The PLISTs differ a lot between the platforms. For gathering the initial
data it is easier to just list the files per platform than trying to get
all the conditions right in the first place.

There will be conditions for operating systems, for platforms, for
features, for version-specific include files to be fixed, for installed
programming languages, for nls, and several more.