Update to the 20201114 snapshot of GCC 10.2.1.

This brings two backports for GOMP, one for aarch64, two for powerpc (nee
rs6000) and two for x86, one for the tree optimizers, two for the C front
end and three for C++ and Fortran each, plus half a dozen for libstdc++.

Forward port part of r555542 | gerald | 2020-11-17 from lang/gcc9-devel:

Fold the PLUGINS option, which has been on by default, into standard
behavior and remove that option.  It has not been causing any issues
since we enabled it 11 months ago, nor measurable overhead. [1]

PR: 242644 [1]