runtime/interrupt: add cross-chip disable/restore interrupt support

This should cover all supported baremetal targets.

src/runtime/interrupt/interrupt_avr.go

@@ -0,0 +1,36 @@
+// +build avr
+
+package interrupt
+
+import "device"
+
+// State represents the previous global interrupt state.
+type State uint8
+
+// Disable disables all interrupts and returns the previous interrupt state. It
+// can be used in a critical section like this:
+//
+//     state := interrupt.Disable()
+//     // critical section
+//     interrupt.Restore(state)
+//
+// Critical sections can be nested. Make sure to call Restore in the same order
+// as you called Disable (this happens naturally with the pattern above).
+func Disable() (state State) {
+	// SREG is at I/O address 0x3f.
+	return State(device.AsmFull(`
+		in {}, 0x3f
+		cli
+	`, nil))
+}
+
+// Restore restores interrupts to what they were before. Give the previous state
+// returned by Disable as a parameter. If interrupts were disabled before
+// calling Disable, this will not re-enable interrupts, allowing for nested
+// cricital sections.
+func Restore(state State) {
+	// SREG is at I/O address 0x3f.
+	device.AsmFull("out 0x3f, {state}", map[string]interface{}{
+		"state": state,
+	})
+}