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_fe310.go

@@ -16,8 +16,6 @@ import (
 
 type timeUnit int64
 
-func postinit() {}
-
 //export main
 func main() {
 	// Zero the PLIC enable bits on startup: they are not zeroed at reset.