Handle generated source files more independently from normal source
files. This allows mixing of generated specs with existing source
bodies.

As with the source file list, we filter the list of generated sources,
`$(name)-gens`: All spec (.ads) files that have a respective body file
(.adb), are moved into `$(name)-extra-gens`. This way, we can still
add these files as dependencies to all pre-existing source files.

Change-Id: I5479c436f39088ae9d0ebddf9a97446be0f191f3
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/libhwbase/+/27139
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>