Update to the 20200314 snapshot of GCC 9.3.1 (after there was no
snapshot the week before due to server migration).

This is the first snapshot after the GCC 9.3 release and as such
brings a bunch of further backports for the release, including some
for the tree and RTL optimizers, Arm, rs6000 and x86 (AVX2) backends,
and in addition to one for Fortran some twenty for the C++ front end.