Backport r488673 | andreast | 2018-12-29 from lang/gcc7:

Fix build of GCC on powerpc64.

While building GCC itself we have to use the built GCC libraries to
configure additional parts of GCC and not the libraires from the host.

Install the built 32-bit libraries. This was not done up to now.

And no longer required GCC 5 to build on powerpc64, making this port a
natural choice of a "base" for newer members of the lang/gcc* family.

PR: 235964, 231804
Submitted by: Piotr Kubaj <pkubaj@anongoth.pl>