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_unix.go

@@ -65,8 +65,6 @@ type timespec struct {
 
 var stackTop uintptr
 
-func postinit() {}
-
 // Entry point for Go. Initialize all packages and call main.main().
 //export main
 func main(argc int32, argv *unsafe.Pointer) int {