Bug 1846703 - Backport a single revision from ffmpeg into our vendored copy, to fix the build with recent binutils. r=glandium, a=RyanVM

Because this library is a rather complex, manually trimmed, subset of ffmpeg,
this doesn't use updatebot. We're currently investigating moving to it to at
least fix some manual work needed, but that hasn't happened yet. In lieu of
adding the new patch to a yaml file, I'm modified the Mozilla-specific README.

Differential Revision: https://phabricator.services.mozilla.com/D189751