feature/featuretags: add auto-generated constants for all modular features

So code (in upcoming PRs) can test for the build tags with consts and
get dead code elimination from the compiler+linker.

Updates #12614

Change-Id: If6160453ffd01b798f09894141e7631a93385941
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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