Ignore __stack_chk_fail* in the rtld mapfile computation [BZ #7065]

The previous commit prevented rtld itself from being built with
-fstack-protector, but this is not quite enough.  We identify which
objects belong in rtld via a test link and analysis of the resulting
mapfile.  That link is necessarily done against objects that are
stack-protected, so drags in __stack_chk_fail_local, __stack_chk_fail,
and all the libc and libio code they use.

To stop this happening, use --defsym in the test librtld.map-production
link to force the linker to predefine these two symbols (to 0, but it
could be to anything).  (In a real link, this would of course be
catastrophic, but these object files are never used for anything else.)