It turns out none of this stuff is needed. The docs aren't evenly built
properly anyways so the build trivially succeeds either way, due to what
looks like upstream misunderstanding automake. If I try to build the
docs manually in a cross shell (before and after this change), there's a
make rule error such that some HTML files aren't even attempted to be
built and then a copy fails.
Even if this was all fixed, these been a good number of cross fixes
upstream getting them to use CC_FOR_BUILD and other good stuff, so I
doubt such hacks would be needed.
Progress towards #40531 and #33302.
29b62e07d6