build system: use platform specific ar(1) for libverstage

Shouldn't be necessary, doesn't hurt either.

Change-Id: I4fa5cc2931523b5beac5ea5126e3e8b841446017
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: http://review.coreboot.org/10140
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
diff --git a/Makefile.inc b/Makefile.inc
index 06c3302..79cfa72 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -365,7 +365,7 @@
 
 $(objgenerated)/libverstage.a: $$(libverstage-objs)
 	rm -f $@
-	ar rcsT $@ $^
+	$(AR_libverstage) rcsT $@ $^
 
 #######################################################################
 # Clean up rules