Update to the 20200215 snapshot of GCC 9.2.1.

This brings many of the changes the 20200214 snapshot of GCC 8.3.1
gained with revision 526353 (which was backports for a RHEL production
release). Overall expanded OMP isupport for Fortran, some four issues
fixed for the tree optimizers, the middle end beyond that, and the C++
front end, and a bit on the AVX/SEE front for amd64.