[Ada] AI12-0364 Add a modular atomic arithmetic package

[Pierre-Marie de Rodat]

This new Ada 202x AI introduces a new package Modular_Arithmetic.
Related discussion also suggested to rename the recently introduced
Arithmetic package -> Integer_Arithmetic, for consistency, so this is
done at the same time.

Tested on x86_64-pc-linux-gnu, committed on trunk

2020-06-10  Arnaud Charlet  <char...@adacore.com>

gcc/ada/

        * libgnat/s-aomoar.ads, libgnat/s-aomoar.adb: New files.
        * libgnat/s-atopar.ads: Move...
        * libgnat/s-aoinar.ads: Here.
        * libgnat/s-atopar.adb: Move...
        * libgnat/s-aoinar.adb: Here.
        * impunit.adb: Update list of runtime files.
        * Makefile.rtl (GNATRTL_NONTASKING_OBJS=): Adjust.

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