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>

Author: cherrymui

src/runtime/signal_unix.go

@@ -350,17 +350,6 @@ const preemptMSupported = true
 // safe-point, it will preempt the goroutine. It always atomically
 // increments mp.preemptGen after handling a preemption request.
 func preemptM(mp *m) {
-	if (GOOS == "darwin" || GOOS == "ios") && GOARCH == "arm64" && !iscgo {
-		// On darwin, we use libc calls, and cgo is required on ARM64
-		// so we have TLS set up to save/restore G during C calls. If cgo is
-		// absent, we cannot save/restore G in TLS, and if a signal is
-		// received during C execution we cannot get the G. Therefore don't
-		// send signals.
-		// This can only happen in the go_bootstrap program (otherwise cgo is
-		// required).
-		return
-	}
-
 	// On Darwin, don't try to preempt threads during exec.
 	// Issue #41702.
 	if GOOS == "darwin" {