stack-protector configure test and MIPS64

[Roland McGrath]

> So, is the patch OK then, as a patch known to improve things for at least 
> one system in practice and not known to make things worse for any compiler 
> people are actually trying to use to build glibc?

Yes.  It would be better with some comments about why we're filtering out
all other symbols and about the general fragility of the situation.