begin isolating target-specific USB code

src/machine/usb/util_arm.go

@@ -4,6 +4,9 @@ package usb
 
 import "device/arm"
 
+//go:linkname ticks runtime.ticks
+func ticks() int64
+
 // udelay waits for the given number of microseconds before returning.
 // We cannot use the sleep timer from this context (import cycle), but we need
 // an approximate method to spin CPU cycles for short periods of time.
@@ -14,3 +17,11 @@ func udelay(microsec uint32) {
 		arm.Asm(`nop`)
 	}
 }
+
+func disableInterrupts() uintptr {
+	return arm.DisableInterrupts()
+}
+
+func enableInterrupts(mask uintptr) {
+	arm.EnableInterrupts(mask)
+}