Place the plugin metadata into special sections for Mac and Win

The plugin metadata is placed into a special section in the
object file on ELF systems to allow finding it fast. Place
the data into a special section on Mac and Windows as well,
even though we don't have Mach-O or COFF parsers available
yet.

This will allow us to add the Mach-O and COFF parsers as a pure
optimization later on.

Task-number: QTBUG-25054
Change-Id: I376d5443ede715aee2c68fb62d24afdbf66e5685
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>