toolchain: Always use GCC for Ada sources

We can't use $(CC) in case it's set to Clang.

TEST=Built one target with Ada sources before and after this change and
     verified that the same compiler commands are emitted.

Change-Id: I9b8ea35352d74b364f09fc12d8d981ca42f8b7c8
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/21366
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/lib/gnat/Makefile.inc

@@ -24,7 +24,7 @@ ADAFLAGS_libgnat-$(1) := \
 	-gnatg \
 	-gnatpg \
 	-I$$(src)/lib/gnat/ \
-	$$(CFLAGS_$(1)) \
+	$$(GCC_CFLAGS_$(1)) \
 
 libgnat-$(1)-y += a-unccon.ads
 libgnat-$(1)-y += ada.ads