[v3] use -Wl,-z,relro if possible

[Paolo Carlini]

Benjamin Kosnik wrote:
>> the issue is by now rather clear, I think, see libstdc++/25797.
>>     
> Thanks guys. I tweaked the configure tests as follows, hopefully this will solve it.
Thanks Benjamin for persisting. Indeed, I'm confident that one of the 
problems, that with gc-sections completely ignored on some targets, is 
fixed.

However, the new configure test is not yet sufficiently strong to 
disable the use of gc-sections on my binutils2.16.1 system. I'm 
available to test further improvements to the configure tests, if you 
like (remember our timezones, though ;)

Thanks,
Paolo.