all: move bootstrapping IR to Go runtime

This has the benefit of not requiring a 'runtime' IR file, so that
complete relocatable files can be built without requiring input IR.
This makes the compiler a lot easier to use without the Makefile.

Code size is not affected.

src/runtime/runtime_nrf.go

@@ -11,6 +11,11 @@ func _Cfunc_rtc_sleep(ticks uint32)
 
 const Microsecond = 1
 
+//go:export _start
+func _start() {
+	main()
+}
+
 func init() {
 	initUART()
 	initLFCLK()