Add minimal GNAT run time system (RTS)

Add a stripped-down version of libgnat. This is somehow comparable to
libgcc but for Ada programs. It's licensed under GPLv3 but with the
runtime library exception. So it's totally fine to link it with our
GPLv2 code and keep it under GPLv2.

Change-Id: Ie6522abf093f0a516b9ae18ddc69131bd721dc0c
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/11836
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Edward O'Callaghan <edward.ocallaghan@koparo.com>

util/lint/lint-stable-009-old-licenses

@@ -18,6 +18,7 @@
 
 # regex list of files and directories to exclude from the search
 HEADER_EXCLUDED="\
+^src/lib/gnat/|\
 ^src/vendorcode/|\
 ^util/kconfig/|\
 \<COPYING\>|\