base: avoid superfluous postprocessing of ldso

Even though the binary patching of ldso must be performed only once,
this postprocessing step was executed on each run because the
postprocess.tag rule is phony (the tag file is never created).
This patch removes this phony behavior by creating the tag file.

Issue #3974

repos/base/lib/mk/ld-platform.inc

@@ -46,6 +46,7 @@ postprocess.tag: $(LIB).lib.so
 	$(VERBOSE)printf "\x02" |\
 	          dd of=$(LIB).lib.so bs=1 seek=16 count=1 conv=notrunc \
 	          2> /dev/null
+	$(VERBOSE)touch $@
 endif
 
 else