Update to the 20210219 snapshot of GCC 9.3.1.

This only brings a minor tweak to the data flow engine.

Plus a back port of part of r565301 | gerald | 2021-02-15 from
lang/gcc11-devel:

Explicitly build --without-zstd such that archivers/zstd isn't pulled
in inadvertedly when present in the build system even though it is not
an explicit dependency. [1]

PR: 253286 [1]