powerpc switched back to GCC 8

On Wed, Jan 22, 2020 at 16:04:46 +0300, Valery Ushakov wrote:

> GCC 8 on powerpc has broken alloca() - http://gnats.netbsd.org/54827
> 
> As there's no fix yet (there's a kludge, but I'm not happy with it) I
> have reverted powerpc to GCC 7 for now.  Ok mrg@.

There's no better way to find a fix for some piece of code than to
disable it...

I believe GCC 8 alloca() is now properly fixed.  Details in the PR.
I have switched powerpc ports back to GCC 8. Ok mrg@.

-uwe