Update to the 20180303 snapshot of GCC 9.0.1.
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]