Update to the 20180301 snapshot of GCC 8.3.1.

Simplify the creation of the multilib-related sub-directory tree on
powerpc64 and avoid leaving an empty directory behind on the way. [1]

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

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