vendorcode: Use cross archiver for libagesa.a on AMD f14

Change-Id: I61a9f65a1ac4c95096d0ff071a95984cf219caa8
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/10593
Reviewed-by: Marc Jones <marc.jones@se-eng.com>
Tested-by: build bot (Jenkins)

src/vendorcode/amd/agesa/f14/Makefile.inc

@@ -326,7 +326,7 @@ libagesa-y += Proc/Mem/Main/mu.c
 
 $(obj)/libagesa.fam14.a: $$(libagesa-objs)
 	@printf " AGESA $(subst $(obj)/,,$(@))\n"
-	ar rcs $@ $+
+	$(AR_romstage) rcs $@ $+
 
 romstage-libs += $(obj)/libagesa.fam14.a
 ramstage-libs += $(obj)/libagesa.fam14.a