mesa: split out the Darwin build into a separate expression, heavily clean up Linux

The way we build Mesa for Linux (and potentially others) is very,
very different from the way we build Mesa on Darwin. In short,
on Linux we generally want to build as much of it as possible,
while on Darwin we want to build just enough of it to get GLX
for legacy X applications.

So, split out the Darwin build into its own expression entirely,
and give it just enough inputs and just enough feature flags to
actually do the one thing we want it to do, and nothing else.

The Linux expression can be simplified further now, but that
would cause rebuilds, so we'll have to do that later.

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