Enabling async preemption on darwin/arm and darwin/arm64 causes
the builder to fail, e.g.
https://build.golang.org/log/03f727b8f91b0c75bf54ff508d7d2f00b5cad4bf

Due to the limited resource, I haven't been able to get access on
those devices to debug. Disable async preemption for now.

Updates #35439.

Change-Id: I5a31ad6962c2bae8e6e9b8303c494610a8a4e50a
Reviewed-on: https://go-review.googlesource.com/c/go/+/205842Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>