[Ada] Rename runtime variant files under libgnarl

[Arnaud Charlet <charlet at adacore dot com>]

This is the next step in the source file and directory reorg in GNAT sources.
This time, we rename all files of the form ?-[a-z]*-* into ?-[a-z]*_*,
for example s-tpopsp-posix.adb becomes s-tpopsp__posix.adb. This is done
so that it's easier to see which files are platform specific variants (such
files will now always contain a double underscore).

This change is for the libgnarl directory, the next change will do the same
for the libgnat directory.

As with the previous changes, there's a small chance that some target got
forgotten in Makefile.in, but hopefully not! If this is the case, fixing it
is purely mechanical and is preapproved.

The diff is huge and mechanical, I'm only including the part in
gcc-interface/Makefile.in.

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

2017-09-11  Jerome Lambourg  <lambourg@adacore.com>

        * libgnarl: Rename ?-[a-z]*-* into ?-[a-z]*__*
        * gcc-interface/Makefile.in: Take this renaming into account.

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