Fix struct sigaltstack namespace (bug 21517)

[Florian Weimer]

On 05/25/2017 12:32 AM, Joseph Myers wrote:
> +* The stack_t type no longer has the name struct sigaltstack.  This changes
> +  the C++ name mangling for interfaces involving this type.

I checked Fedora and we don't seem to have any ABI impact from this
change (no exported symbols matching /^_Z.*sigaltstack/), so this is
probably fine.

Thanks,
Florian