Update to the 20200509 snapshot of GCC 10.1.1.

This is the first snapshot after the GCC 10.1 release, the first in the
series, so our version moves from 10.0.1 to 10.1.1.

It brings three optimizer fixes/improvements, one for the aarc64 and
two for the x86-64 back end, and some ten for C/C++.