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_arm7tdmi.go

@@ -28,8 +28,6 @@ var _sidata [0]byte
 //go:extern _edata
 var _edata [0]byte
 
-func postinit() {}
-
 // Entry point for Go. Initialize all packages and call main.main().
 //export main
 func main() {