powerpc reverted to GCC 7

On Wed, Jan 01, 2020 at 23:55:36 +0300, Valery Ushakov wrote:

> I have upgraded my test mac mini g4 to current from Dec 22 but got
> around to compile something on it last night only and it looks like
> gcc8 is busted (powerpc ports were switched on Dec 14).
> 
> $ cc --version | sed 1q
> cc (nb1 20190930) 8.3.0
> $ cat main.c
> int main() { return 0; }
> $ /usr/libexec/cc1 main.c
> <built-in>: internal compiler error: Segmentation fault

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@.

If you grok PPC ABIs and GCC internals, help with that PR would be
much appreciated.

-uwe