Using System.Machine_Code in GPL 2017 Zfp for Arm

[ahlan@marriott.org]

We are trying to build a zfp system for an arm M3 processor that is not on the list of supported GPL processors.
We only want the Ada intrinsics. We do not need or want any RTS and so expected that --RTS=zfp would do the trick.
however when we place this switch on arm-eabi-gcc from GPL 2017 we get the error message
RTS path not valid: Missing adainclude and adalib directories.
So things are not as simple as we thought.
It appears that --RTS takes the name of a directory so we created one and named this on the RTS switch.
In the directory we placed two files ada_object_path and ada_source_path that both contain the single word gnat.
In the RTS directory we then created the empty directory adalib and another  directory gnat and placed in gnat directory the file System.ads which we copied from an RTS directory of a similar processor.
This all seems to work.
With this we are able to create simple programs.
However when we want to use System.Machine_Code we get the error message that
System.Machine_Code is not a predefined library.
I suspect that we need to add something to our RTS directory.
But what?
Has anyone any experience building ARM ZFP systems and can tell us how we get add System.Machine_Code to our Zfp.
Any help or suggestions what to do next would be greatly appreciated.

Best wishes,
Ahlan