Add machine.CPUReset() (#3595)

machine: Add machine.CPUReset() for cortexm

src/machine/machine_cortexm.go

@@ -0,0 +1,10 @@
+//go:build cortexm
+
+package machine
+
+import "device/arm"
+
+// CPUReset performs a hard system reset.
+func CPUReset() {
+	arm.SystemReset()
+}