The problem should be fixed by the previous CL. Reenable async
preemption on darwin/arm64.

Updates #35439.

Change-Id: I93e8c4702b4d8fe6abaa6fc9c27def5c8aed1b59
Reviewed-on: https://go-review.googlesource.com/c/go/+/206419Reviewed-by: Keith Randall <khr@golang.org>