runtime: use common library entry point on windows amd64/386

runtime: use common library entry point on windows amd64/386

Windows can reuse the common library entry point instead of implementing
a its own version. Note that windows/arm64 already uses the common one.

Change-Id: I1a27bbec04bfd1d58a136638bafcdc0583bd106f
Reviewed-on: https://go-review.googlesource.com/c/go/+/706235
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>