implement efficient waiting on a Cond without a scheduler

Author: niaow

src/runtime/arch_cortexm.go

@@ -106,3 +106,7 @@ func procUnpin() {
 func waitForEvents() {
 	arm.Asm("wfe")
 }
+
+func pollWait() {
+	arm.Asm("wfi")
+}

src/runtime/arch_tinygoriscv.go

@@ -88,3 +88,7 @@ func waitForEvents() {
 	}
 	riscv.EnableInterrupts(mask)
 }
+
+func pollWait() {
+	riscv.Asm("wfi")
+}

src/runtime/cond_nosched.go

@@ -34,6 +34,9 @@ func (c *Cond) Poll() bool {
 // Wait for a notification.
 // If the condition variable was previously notified, this returns immediately.
 func (c *Cond) Wait() {
-	for !c.Poll() {
+	i := interrupt.Disable()
+	for !c.fired {
+		pollWait()
 	}
+	interrupt.Restore(i)
 }

src/runtime/wait_other.go

@@ -6,3 +6,7 @@ package runtime
 func waitForEvents() {
 	runtimePanic("deadlocked: no event source")
 }
+
+func pollWait() {
+	runtimePanic("deadlocked: no event source")
+}