Mk/Uses/ada.mk: Don't set CC to gnatmake but to gcc

Setting CC to gnatmake breaks compiling simple C objects. Instead
set it to the correct compiler frontend and set GNATMAKE to gnatmake.

Signed-off-by:	Nico Sonack <nsonack@herrhotzenplotz.de>
PR:		274238

Mk/Uses/ada.mk

@@ -40,7 +40,8 @@ CC=		ada
 _ADAPATH=	${LOCALBASE}/gnat${_ada_version}/bin
 _ADAPORT=	lang/gnat${_ada_version}
 _BINDEP=	${_ADAPATH}/gnat
-CC=		gnatmake -f -u
+GNATMAKE=	gnatmake -f -u
+CC=		gcc
 BINARY_ALIAS+=	ada=${_ADAPATH}/gcc
 .  endif