avr: fix time.Sleep() in init code

In the early days of TinyGo, the idea of `postinit` was to enable
interrupts only after initializers have run. Which kind of makes
sense... except that `time.Sleep` is allowed in init code and
`time.Sleep` requires interrupts to be enabled. Therefore, interrupts
must be enabled while initializers are being run.

This commit simply moves the enabling of interrupts to a point right
before running package initializers. It also removes `runtime.postinit`,
which is not necessary anymore (and was only used on AVR).

src/runtime/runtime_nxpmk66f18.go

@@ -226,8 +226,6 @@ func initInternal() {
 	// 	analog_init();
 }
 
-func postinit() {}
-
 func putchar(c byte) {
 	machine.PutcharUART(machine.UART0, c)
 }