runtime: disable async preemption on darwin/arm(64) for now

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>

src/runtime/signal_arm.go

@@ -63,7 +63,9 @@ func (c *sigctxt) preparePanic(sig uint32, gp *g) {
 	c.set_pc(uint32(funcPC(sigpanic)))
 }
 
-const pushCallSupported = true
+// TODO(issue 35439): enabling async preemption causes failures on darwin/arm.
+// Disable for now.
+const pushCallSupported = GOOS != "darwin"
 
 func (c *sigctxt) pushCall(targetPC uintptr) {
 	// Push the LR to stack, as we'll clobber it in order to

src/runtime/signal_arm64.go

@@ -79,7 +79,9 @@ func (c *sigctxt) preparePanic(sig uint32, gp *g) {
 	c.set_pc(uint64(funcPC(sigpanic)))
 }
 
-const pushCallSupported = true
+// TODO(issue 35439): enabling async preemption causes failures on darwin/arm64.
+// Disable for now.
+const pushCallSupported = GOOS != "darwin"
 
 func (c *sigctxt) pushCall(targetPC uintptr) {
 	// Push the LR to stack, as we'll clobber it in order to