buildgcc: Ask the user to install gnat if it's missing

Change-Id: Ib840eac29fc8cedfaef4847fd9700bd4a70300ba
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/16677
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/crossgcc/buildgcc

@@ -241,6 +241,8 @@ check_gnat() {
 			printf "requires bootstrapping (-b).${NC}\n\n"
 			HALT_FOR_TOOLS=1
 		fi
+	else
+		please_install gnat
 	fi
 }