[libstdc++; pr66689; pr68397] Backport special function fixes to 7.

[Jonathan Wakely]

On 26/04/18 23:17 -0400, Ed Smith-Rowland wrote:
>I'm thinking of going back on my choice not to fix special function 
>bugs on 7.
>
>These build and test clean on gcc-7.
>
>OK?

OK for gcc-7-branch, thanks.