No longer require a not too old version of GCC to build on powerpc64, but
rely on the system compiler (even if that means we need to be explicitly
conservative when it comes to optimizations). [1]

Simplify the creation of the multilib-related sub-directory tree on
powerpc64 and avoid leaving an empty directory behind on the way. [2]

Sync pkg-descr with lang/gcc7-devel, in particular after r442530 there.

PR: 235975 [1], 235964 [2], 231804 [2]
Submitted by: Piotr Kubaj <pkubaj@anongoth.pl> [1]
Discussed with: Piotr Kubaj <pkubaj@anongoth.pl> [2]