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_tinygowasm.go

@@ -22,8 +22,6 @@ func fd_write(id uint32, iovs *__wasi_iovec_t, iovs_len uint, nwritten *uint) (e
 //export proc_exit
 func proc_exit(exitcode uint32)
 
-func postinit() {}
-
 const (
 	putcharBufferSize = 120
 	stdout            = 1