build: add profile-guided optimization (PGO)

[ee7]

See https://en.wikipedia.org/wiki/Profile-guided_optimization

PGO is great, but configlet currently doesn't do anything that's particularly slow.

There's also potential for reducing binary size, though. And I'm always interested in low-hanging fruit there.

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Low priority. I'm just creating some issues to document some things I'm aware of, but I have zero intention of doing (at least right now).