Compatibility to GNAT Community 2021

[treiher]

Compiling the runtime with GNAT Community 2021 leads to an error:

% make
make SRC_COMMON="a-except.adb a-unccon.ads ada.ads a-tags.ads a-tags.adb a-numeri.ads a-nubinu.ads a-nbnbin.adb i-c.adb i-cexten.ads interfac.ads componolit.ads componolit-runtime.ads componolit-runtime-strings.adb componolit-runtime-debug.adb componolit-runtime-platform.adb componolit-runtime-exceptions.ads componolit-runtime-conversions.adb s-exctab.adb s-init.adb s-parame.adb s-unstyp.ads s-secsta.adb s-soflin.adb s-stalib.adb s-stoele.adb s-arit64.adb s-maccod.ads system.ads argv.c exit.c init.c componolit_runtime.h ada_exceptions.h gnat_helpers.h" -C build/posix
make[1]: Entering directory '/tmp/ada-runtime/build/posix'
make[1]: *** No rule to make target '../../src/minimal/a-tags-gcc_10.ads', needed by 'obj/adainclude/a-tags.ads'.  Stop.
make[1]: Leaving directory '/tmp/ada-runtime/build/posix'
make: *** [Makefile:46: posix] Error

[jklmnn]

Fixed by #81.