Proper way to license a ZFP runtime

[Jere <jhb.chat@gmail.com>]

I'm building up a ZFP runtime targeting the GNAT compiler and
had some questions about how to handle licensing the files.  Take a
package like System.Storage_Elements.  The Ada RM leaves quite a 
few of the items as "implementation defined" (which makes sense), 
and I have to add implementations for the functions, but since I 
am targeting the GNAT compiler, I will want to use GNAT specific
language extensions to implement them.  Does this force me to
use one of the GNAT licenses?  

Additionally, defining some of the types in the package System 
are also "implementation defined" but there really is only one 
way to define them for a board of similar type (32bit arm).  
Since they have already defined in runtimes for other similar
arm boards, am I forced to use the copywrite and license of the
runtimes for those boards?

If I don't have to use the licenses mentioned above, is there
a good license statement that conveys the files are simply
based on the Ada RM and are free to use however one wants?
I feel like I have seen one before in a repo, but couldn't 
locate it, so it might have just been in my head.

Same type of questions apply to copyright as well.

If I have to, I'll put the GNAT copyright and license on all
the files, but I would prefer to only do that on files that
I include or modify from actual existing GNAT files