Update to the 20201226 snapshot of GCC 10.2.1.

This brings a backport for the RTL optimizers, two for the x86 back end,
two for the C++ front end, and one for Fortran.

It also changes the processor defaults for 32- und 64-bit powerpc to
PPC7450 and POWER8, respectively, per a request by pkubaj@ that I pushed
upstream. [1]

PR: 251670 [1]