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_arm.go

@@ -40,9 +40,6 @@ func preinit() {
 	}
 }
 
-func postinit() {
-}
-
 func abort() {
 	for {
 		arm.Asm("wfi")