runtime: refactor initialization code

Let each target handle its own initialization/finalization sequence
instead of providing one in the runtime with hooks for memory
initialization etc. This is much more flexible although it causes a
little bit of code duplication.

src/runtime/runtime_stm32.go

@@ -11,11 +11,11 @@ type timeUnit int64
 const tickMicros = 1 // TODO
 
 //go:export Reset_Handler
-func handleReset() {
-	main()
-}
-
-func init() {
+func main() {
+	preinit()
+	initAll()
+	mainWrapper()
+	abort()
 }
 
 func putchar(c byte) {