No, but anyone who's that interested CAN have one of these in their garage (or on their desk, more likely), and get their design fabbed by these guys fairly cheaply.
Sure, it's not quite as easy as hacking on open source, but hobbyist CPU design is definitely possible. Especially when you consider there ARE open source CPU designs out there.
Then you're missing the point of Gentoo, which is to let you do whatever the hell you want.
One of the ways this is achieved is compiling almost everything from source, with your own configure compiler options. Except it makes doing that really really easy; once you've done things the Gentoo way everything else seems to be really inflexible, full of busywork or both.
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