crossgcc: Preparations for building Ada frontend

crossgcc: Preparations for building Ada frontend

As with most other languages, a pre-installed Ada toolchain is needed
to build gcc's Ada frontend. To support building with older host tool-
chains, the patch `gcc-5.2.0_gnat.patch` disables warnings for unknown
pragmas. Building has been tested with host gcc-4.9 and hopefully works
with newer versions, too.

For convenience, the gnattools (e.g. gnatmake etc.) will be build if
'ada' is specified as a target language.

Change-Id: Ia78c29d1aba2943de5238421a324cfff8eb08875
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: http://review.coreboot.org/11590
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>