Emit a warning when -z relro is unsupported

[Joseph Myers]

On Wed, 29 Jul 2020, Florian Weimer via Libc-alpha wrote:

> This change looks reasonable to me.  Thanks for tracking this down.
> 
> We are in the glibc 2.32, so Carlos as the release manager needs to
> approve this.

I confirm this patch gets clean test results for build-many-glibcs.py 
(compilers and glibcs) with GCC 9 branch (where previously ia64 failed).

-- 
Joseph S. Myers
joseph@codesourcery.com