Update to the 20200125 snapshot of GCC 9.2.1.

This brings backports for fixes in the aarch64, i386, and rs6000 (alias
powerpc) backends, Fortran and OMP plus half a dozen C++ issues, and loop,
inter-process and some other optimizations issues. In other words, more
work largely done for a big distribution using GCC 9 as system compiler.