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_esp32xx.go

@@ -14,8 +14,6 @@ func putchar(c byte) {
 	machine.Serial.WriteByte(c)
 }
 
-func postinit() {}
-
 // Initialize .bss: zero-initialized global variables.
 // The .data section has already been loaded by the ROM bootloader.
 func clearbss() {