runtime: generate windows callback list with go generate

This is the last system-dependent file written by cmd/dist.
They are all now written by go generate.

cmd/dist is not needed to start building package runtime
for a different system anymore.

Now all the generated files can be assumed generated, so
delete the clumsy hacks in cmd/api.

Re-enable api check in run.bash.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/185040044