cmd/tailscaled, net/tstun: build for aix/ppc64

At least in userspace-networking mode.

Fixes #11361

Change-Id: I78d33f0f7e05fe9e9ee95b97c99b593f8fe498f2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f