kbuild: Fix copying of shipped files

When using cp to copy the shipped file to its actual name,
permissions would be preserved, particularly the copy would be
read-only when the original was (BitKeeper) read-only, leading
to an error when executing the rule a second time.

So now we use cat, which will generate a writable file.

Rules.make

@@ -412,7 +412,7 @@ endif # ! fastdep
 # ===========================================================================
 
 quiet_cmd_shipped = SHIPPED $(echo_target)
-cmd_shipped = cp $< $@
+cmd_shipped = cat $< > $@
 
 %:: %_shipped
 	$(call cmd,shipped)