Rollup merge of #49187 - alexcrichton:no-cross-docs, r=kennytm

rustbuild: Disable docs on cross-compiled builds

This commit disables building documentation on cross-compiled compilers, for
example ARM/MIPS/PowerPC/etc. Currently I believe we're not getting much use out
of these documentation artifacts and they often take 10-15 minutes total to
build as it requires building rustdoc/rustbook and then also generating all the
documentation, especially for the reference and the book itself.

In an effort to cut down on the amount of work that we're doing on dist CI
builders in light of recent timeouts this was some relatively low hanging fruit
to cut which in theory won't have much impact on the ecosystem in the hopes that
the documentation isn't used too heavily anyway.

While initial analysis in #48827 showed only shaving 5 minutes off local builds
the same 5 minute conclusion was drawn from #48826 which ended up having nearly
a half-hour impact on the bots. In that sense I'm hoping that we can land this
and test out what happens on CI to see how it affects timing.

Note that all tier 1 platforms, Windows, Mac, and Linux, will continue to
generate documentation.

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