runtime: unify arm64 entry point code

runtime: unify arm64 entry point code

There is a lot of duplication in how arm64 OSes handle entry points.
Do as amd64, have all the logic in a common function.

Cq-Include-Trybots: luci.golang.try:gotip-darwin-arm64-longtest,gotip-windows-arm64
Change-Id: I370c25c3c4b107b525aba14e9dcac34a02d9872e
Reviewed-on: https://go-review.googlesource.com/c/go/+/706175
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
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