Update to the 20201121 snapshot of GCC 10.2.1.

This brings a number of backports for aarch64 and two for x86, plus
thirteen for libstdc++.

When building for powerpc with the old ELF ABI, lock down to USE_GCC=9
instead of USE_GCC=yes since GCC 10 is going to become that new default
shortly, which would cause a dependency on ourselves otherwise. [1]

(This also addresses one of the few differences between gcc10-devel and
gcc10 we want to unify.)

PR: 246700 [1]