runtime: always enable async preemption on darwin/arm64

Now that we have the G register saved, we can enable asynchronous
preemption for pure Go programs on darwin/arm64.

Updates #38485, #36365.

Change-Id: Ic654fa4dce369efe289b38d59cf1a184b358fe9e
Reviewed-on: https://go-review.googlesource.com/c/go/+/265120


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>