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).

main_test.go

@@ -171,9 +171,6 @@ func TestCompiler(t *testing.T) {
 			case "float.go", "math.go", "print.go":
 				// Stuck in runtime.printfloat64.
 
-			case "goroutines.go":
-				// The main() never runs.
-
 			case "interface.go":
 				// Several comparison tests fail.